Most SIP trunk contracts get signed on the strength of a rate card and a sales call. The problems show up later: a channel cap you did not know existed, B-level attestation because the provider signs through someone else, a billing increment that quietly inflates every invoice. By then you are mid-campaign and switching costs are real.
The fix is asking the right questions before you sign — and knowing what a bad answer sounds like. This matters most for high-volume outbound operations, where the trunk is not a commodity input: it determines answer rate, abandon rate, agent utilization, and revenue per hour. A predictive dialer with 50 agents can be undone entirely by a SIP trunking contract that looked fine on paper.
Here are the seven questions, why each one matters, and the answers that should end the conversation.
The seven questions
1. Concurrent channel policy
Question to ask: “Is there a per-trunk or per-account concurrent channel limit? What happens when I reach it?”
Why it matters: A predictive dialer with 50 agents running a 4:1 ratio needs 200 concurrent channels. With answer rate drops, burst demand can hit 250 to 300. If your provider caps at 50 or 100 channels, your dialer receives SIP 503 errors on every call above the cap. The algorithm cannot execute its predictions. Agents sit idle while the provider throttles your capacity.
What to look for: “Unlimited” or a cap significantly higher than your peak burst requirement. If the provider charges per channel, calculate the cost at your peak concurrency — 200 channels at $1/channel/month adds $200 to your bill for capacity you only burst to intermittently.
2. STIR/SHAKEN attestation level
Question to ask: “Do you hold your own STIR/SHAKEN Service Provider certificate, or do you sign through an upstream carrier? What attestation level will my calls receive?”
Why it matters: A-level attestation provides the highest trust signal to terminating carriers and analytics companies. B-level — the typical result when a reseller’s upstream carrier signs on your behalf — provides no protective benefit and can contribute to spam labeling. The answer rate difference between A and B level is typically 10 to 20 percent on the same leads.
What to look for: The provider holds its own SP-KI certificate and signs directly. They verify your DIDs for A-level rather than passing your traffic upstream for B-level signing.
3. Post-dial delay (PDD)
Question to ask: “What is your average PDD for US domestic outbound? What is the 95th percentile?”
Why it matters: PDD is the time between your SIP INVITE and the recipient’s phone ringing. Predictive dialing algorithms assume relatively consistent call setup times. If PDD varies from 2 to 8 seconds across calls, the algorithm over-dials on some cycles (causing abandons) and under-dials on others (causing idle time). Target: under 3 seconds average, under 5 seconds at 95th percentile.
What to look for: A provider that monitors and publishes PDD metrics. If they cannot tell you their PDD numbers, they do not optimize for them.
4. Codec support
Question to ask: “Do you support G.711u (ulaw) as the primary codec? Do you transcode or pass through natively?”
Why it matters: G.711u is the standard codec for call center quality — 64 kbps uncompressed audio with no encoding artifacts. VICIdial’s AMD (Answering Machine Detection) performs best on G.711 because the audio analysis algorithms are tuned for uncompressed waveforms. Transcoding (converting between codecs at the carrier level) adds latency and can introduce quality degradation. Native pass-through is preferred.
What to look for: G.711u as the default, G.729 as a fallback, no mandatory transcoding. Some providers force G.729 on all traffic to save bandwidth on their network — this saves them money at the cost of your audio quality.
5. Pricing transparency
Question to ask: “Can I see your rate card without talking to a salesperson?”
Why it matters: Hidden pricing means price discrimination — you pay what the sales team thinks you will accept, not what the service is worth. Transparent pricing means you can compare on an apples-to-apples basis.
What to look for: Published per-minute rates, clearly stated billing increments (6-second is standard), no per-channel fees, no platform fees, no setup fees, no porting fees. If anything is missing from the public pricing, ask specifically about it — undisclosed fees are the most common source of billing surprises.
6. Support quality
Question to ask: Call the support number and ask: “What codec and DTMF mode do you recommend for VICIdial’s AMD detection?”
Why it matters: If support cannot answer this question without escalating, they do not understand your workload. When your campaign goes sideways at 2 PM on a Tuesday — answer rates cratered, one-way audio on half your calls, agents hearing echo — you need someone who can diagnose the problem in real time, not someone who opens a ticket and promises a 24-hour response.
What to look for: First-call resolution on dialer-specific questions. Familiarity with VICIdial, Asterisk, FreePBX. Ability to check SIP traces, media quality metrics, and attestation status while you are on the phone.
7. Network compliance
Question to ask: “Are you registered in the FCC Robocall Mitigation Database? Can I verify your filing?”
Why it matters: Providers not registered in the RMD may have their traffic blocked by downstream carriers. Your calls never reach the terminating network. This is an invisible problem — your dialer shows the call as sent, but the recipient’s phone never rings.
What to look for: Active RMD filing, verifiable at fccprod.servicenowservices.com/rmd. Complete STIR/SHAKEN implementation (not just a mitigation program).
Red flags to watch for
“Contact sales for pricing.” They are planning to overcharge you.
Per-channel fees. Artificial scarcity that penalizes predictive dialing.
1-minute billing increments. Inflates your costs by 20 to 30 percent on short calls.
“We partner with [upstream carrier] for STIR/SHAKEN.” Your calls get B-level.
Long-term contracts with early termination fees. They expect you to want to leave.
Support only via ticket/email. When your campaign is down, a 24-hour ticket response costs you thousands in lost revenue.
How SIPNEX answers these questions
We publish these seven questions because we are comfortable being measured against them. Point by point: no channel limit and no per-channel fee — your capacity is your bandwidth and server. Our own SP-KI certificate, A-level attestation on all verified DIDs, direct signing with no intermediate carrier. Sub-3-second average PDD, monitored in real time. Native G.711u pass-through. Published rates with 6-second billing and no hidden fees. Support staff who run VICIdial themselves and answer dialer questions on the first call. Registered in the RMD with complete STIR/SHAKEN implementation.
The full spec — channels, attestation, PDD, codecs, pricing — is on our predictive dialer SIP trunk carrier page.
Frequently asked questions
Which of these questions matters most?
STIR/SHAKEN attestation level has the single largest impact on your bottom line. The difference between A-level (direct carrier) and B-level (reseller) attestation typically represents 10 to 20 percent in answer rate — which translates directly into revenue. Unlimited concurrent channels is the second most important factor, because a channel cap physically prevents a predictive dialer from executing its algorithm. All other factors (PDD, codec, pricing, support) matter, but attestation and channels are the two that most often determine whether a provider is viable for outbound at scale.
Can I test a provider before committing?
You should. Any provider that refuses a trial or proof-of-concept is a provider that does not want you comparing their performance to alternatives. Put both trunks on the same campaign for a week and let the CDRs decide — answer rate, PDD, audio quality, and effective cost per connected minute on identical lead lists. The data will tell you everything the sales call cannot, and a provider confident in its answers to the seven questions above will welcome the comparison.
Should I use one provider or multiple?
Most operations should start with one primary provider and evaluate the need for a secondary. Reasons to use multiple: failover redundancy (if your primary has an outage), cost optimization (route traffic to the cheapest carrier per destination), and A/B testing (compare answer rates between providers). The tradeoff is management complexity — each provider has different credentials, DID inventories, and attestation characteristics. For most mid-size operations, a single reliable provider with carrier-level failover is simpler and sufficient.
Ask these seven questions of any provider you are evaluating — including us. Our answers, with the numbers behind them, are on the predictive dialer SIP trunk carrier page. If you would rather put the questions to a person, reach the SIPNEX team.
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FCC-licensed carrier with its own STIR/SHAKEN SP certificate. Operator-owned. SIP trunks built for operators who dial at volume.