Telecom runs on acronyms. A single support call between an operator and a carrier might include ASR, AHT, PDD, CPS, MOS, CDR, AMD, and CID — and both parties are expected to know what each one means. If you are new to the industry, or if you have been running a call center for years and some of these still blur together, this guide provides clear definitions with operational context. These are not textbook definitions — they are explanations of what each metric means for your daily operation and what to do when the numbers look wrong.
SIPNEX is an FCC-licensed carrier that provides SIP trunks for call centers. Our support conversations use these acronyms daily. When you call us about a problem, we want both sides speaking the same language.
Carrier and network metrics
ASR — Answer-Seizure Ratio. The percentage of call attempts that result in a connection (any answer — human, voicemail, or machine). Calculated as: answered calls ÷ total call attempts × 100. A healthy ASR for US domestic outbound is 40 to 60 percent. Below 40 percent may indicate carrier routing problems, high invalid/disconnected numbers in your list, or network issues. ASR measures network performance, not campaign performance — it tells you whether calls are completing, regardless of whether a human answers. If your ASR drops suddenly without a change in lead source, investigate your carrier. See our call center metrics guide for deeper analysis.
PDD — Post-Dial Delay. The time between your system sending the SIP INVITE and the called party’s phone starting to ring. Measured in seconds. Target: under 3 seconds for US domestic. High PDD breaks predictive dialer algorithms because the pacing predictions assume consistent call setup times. PDD is a carrier-side metric — if it is high or variable, the problem is your carrier’s routing or peering, not your system. On SIPNEX, we target sub-3-second PDD and monitor it in real time.
CPS — Calls Per Second. The rate at which your system can initiate new call attempts. CPS is limited by your carrier’s ingestion capacity, your Asterisk/VICIdial server’s processing power, and your network bandwidth. A typical VICIdial server handles 10 to 30 CPS depending on hardware and configuration. If you hit your carrier’s CPS limit, new SIP INVITEs get queued or rejected (SIP 503 errors).
MOS — Mean Opinion Score. A composite score from 1 (unintelligible) to 5 (excellent) that represents voice call quality. Calculated from latency, jitter, packet loss, and codec characteristics. Target: 4.0+ for call center operations. See our VoIP quality guide for detailed analysis.
CDR — Call Detail Record. A data record generated for every call, containing: call start time, end time, duration, calling number, called number, disposition (answered, no answer, busy, failed), and often additional fields (agent ID, campaign ID, recording path). CDRs are essential for billing reconciliation, compliance documentation, and performance analysis. SIPNEX provides real-time CDR access for all calls.
SER — Seizure-Error Ratio. The percentage of call attempts that result in an error (SIP 4xx, 5xx, or 6xx responses). Target: under 5 percent. High SER indicates network problems, carrier issues, or invalid numbers in your lead list. Common SIP error codes: 404 (number not found), 486 (busy), 503 (service unavailable — often carrier throttling), 603 (decline — call actively rejected).
Agent and campaign metrics
AHT — Average Handle Time. The average total time an agent spends on a call, including talk time and wrap-up (after-call work). AHT = talk time + wrap-up time. Varies enormously by campaign: 2 to 3 minutes for verification calls, 5 to 10 for sales, 15 to 30 for complex consultations. Track AHT by agent to identify outliers — agents with much higher AHT may need coaching on call control; agents with much lower AHT may be cutting calls short.
ACD — Automatic Call Distribution. The system that routes incoming calls to available agents based on rules — skills-based routing, round-robin, longest-idle, queue priority. In VICIdial, ACD functionality is handled by the in-group configuration. ACD is also used as a metric abbreviation for Average Call Duration (distinct from AHT because it excludes wrap-up time).
ACW — After-Call Work. The time agents spend on tasks after disconnecting a call — dispositioning, notes, CRM updates. Also called wrap-up time. Excessive ACW reduces agent utilization. Target: 15 to 30 seconds for simple campaigns, 30 to 60 for complex ones. In VICIdial, configure wrap-up time limits and forced dispositioning to manage ACW.
AMD — Answering Machine Detection. The dialer feature that analyzes the audio in the first few seconds of an answered call to determine whether a human or a machine (voicemail greeting) answered. AMD is essential for predictive dialing efficiency — connecting agents to voicemail wastes time. AMD accuracy depends on codec (G.711 is best), algorithm tuning, and carrier audio quality. False positives (human classified as machine) are lost conversations. False negatives (machine classified as human) waste agent time.
Occupancy Rate. The percentage of agent logged-in time spent on calls (talk + wrap-up) versus idle time (waiting for next call). Target: 85 to 95 percent for predictive dialing, 50 to 70 percent for progressive, 30 to 50 percent for preview. Below target = dialer not pacing aggressively enough or carrier cannot deliver calls fast enough. Above 95 percent = agent burnout risk.
Caller ID and compliance metrics
CID — Caller ID. The phone number displayed to the called party on outbound calls. Set in the SIP From header. CID management — including rotation, local presence, CNAM registration, and reputation monitoring — is one of the most impactful factors in answer rate.
CNAM — Calling Name. The 15-character name string displayed alongside the CID number. Stored in LIDB databases and looked up by the terminating carrier. See our CNAM guide.
DNC — Do Not Call. The National Do Not Call Registry maintained by the FTC, plus company-specific and state DNC lists. Scrub every list against DNC before loading into your dialer. See our TCPA guide.
ATDS — Automatic Telephone Dialing System. The legal classification under TCPA that determines consent requirements. See our auto dialer laws guide for the current legal landscape.
STIR/SHAKEN — Secure Telephony Identity Revisited / Signature-based Handling of Asserted information using toKENs. The cryptographic caller ID verification framework. See our STIR/SHAKEN guide and technical deep dive.
SIP and protocol terms
SIP — Session Initiation Protocol. The signaling protocol for setting up, modifying, and tearing down voice sessions. See our SIP vs VoIP guide.
RTP — Real-time Transport Protocol. The protocol that carries actual voice audio packets during a call. See our RTP guide.
SDP — Session Description Protocol. The protocol embedded in SIP messages that describes the media session parameters — codecs, IP addresses, ports. SDP is how your system and the carrier negotiate which codec to use.
DTMF — Dual-Tone Multi-Frequency. The touch-tone signals generated by pressing phone keys. In SIP, DTMF can be transmitted via RFC 2833 (out-of-band, recommended), SIP INFO, or in-band audio. RFC 2833 is the standard for SIP trunking.
NAT — Network Address Translation. The process of translating private IP addresses to public ones at your router. NAT causes one-way audio problems in VoIP if not configured correctly. See our VICIdial setup guide for NAT configuration.
QoS — Quality of Service. Router-level traffic prioritization that ensures voice packets are processed before data packets. Essential for maintaining call quality on shared internet connections. See our VoIP quality guide.
Number and infrastructure terms
DID — Direct Inward Dial. A phone number that routes directly to a specific endpoint. See our DID guide.
LNP — Local Number Portability. The FCC-mandated right to take your phone numbers to any carrier. See our number porting guide.
NANPA — North American Numbering Plan Administration. The organization that manages area code and number block assignments in North America.
NPA-NXX — Numbering Plan Area - Central Office Code. The first 6 digits of a North American phone number (area code + exchange). Used for routing, rate center identification, and geographic determination.
PRI — Primary Rate Interface. Legacy voice delivery: 23 channels over a physical T1 line. Being decommissioned. See our SIP vs PRI guide.
RespOrg — Responsible Organization. The entity authorized to manage toll-free number routing. See our toll-free guide.
PSTN — Public Switched Telephone Network. The global telephone network infrastructure that connects calls between endpoints.
Frequently asked questions
What is the difference between ASR and answer rate?
ASR (Answer-Seizure Ratio) counts ALL answered calls — human, voicemail, and machine. It measures network completion, not campaign performance. Answer rate (or contact rate) counts only calls answered by a live human. It measures campaign effectiveness. ASR of 50% with AMD filtering might yield an answer rate of 15% (because most answered calls are voicemail). ASR tells you if your carrier and network are working. Answer rate tells you if your campaign is working. Both matter but they measure different things.
What is a good AHT for a call center?
AHT varies by campaign type. Simple verification or appointment confirmation: 2 to 3 minutes. Sales calls: 5 to 10 minutes. Complex consultations (insurance, financial, legal): 15 to 30 minutes. “Good” AHT is relative to your campaign — the goal is not the shortest AHT but the optimal AHT that balances conversation quality with throughput. Track AHT by agent relative to the team average. Agents significantly above average may need coaching on call control. Agents significantly below may be rushing or cutting calls short.
What does PDD mean and why does it matter?
PDD (Post-Dial Delay) is the time between your system sending the call request and the recipient’s phone starting to ring. Target: under 3 seconds for US domestic. PDD matters especially for predictive dialers because the algorithm predicts agent availability based on expected call setup times. Variable PDD (2 seconds on some calls, 8 on others) causes the algorithm to over-dial or under-dial, increasing abandon rates or agent idle time. PDD is determined by your carrier’s routing and peering quality — if PDD is consistently high, talk to your carrier or switch carriers.
SIPNEX speaks the language of operators. When you call our support about ASR drops or PDD spikes, we diagnose at the carrier level without asking you to explain the acronyms. Get a trunk from people who get it or see our rates.
Keep Reading
SIPNEX
FCC-licensed carrier with its own STIR/SHAKEN SP certificate. Operator-owned. SIP trunks built for operators who dial at volume.