Calls-per-second (CPS) limits and your carrier

Calls per second (CPS) is how many new call attempts your carrier will accept each second. Carriers cap it to protect their switches, and when you exceed the cap they start rejecting attempts — usually with a 503 response and congestion tones.

What CPS is

Calls per second (CPS) measures the rate at which you initiate new calls, not how many are live at once. A carrier might allow you 5 CPS — meaning at most 5 fresh call setups per second — even if your concurrent channel limit is much higher. The two limits are independent: one bounds how fast you start calls, the other bounds how many run at the same time.

A Predictive dialing campaign launching a wave of calls can spike well past a low CPS cap in a single second, even though the steady-state channel count stays modest. That's why CPS catches people out — the average looks fine, but the bursts don't.

Why carriers cap it

Setting up a call is the expensive part for a Carrier. Each new attempt over SIP (Session Initiation Protocol) makes their switch do real work — routing lookups, signaling, and downstream connections — before any audio flows. A flood of setups per second can overload their equipment, so carriers cap CPS to keep their network stable for everyone on it.

The cap also discourages abusive dialing patterns. A trunk hammering thousands of setups per second looks like exactly the kind of traffic carriers want to throttle before it causes problems.

Different carriers cap CPS at very different levels. A budget wholesale Trunk might hand you only 1 or 2 CPS, while a higher-tier account negotiated for a call center can run much higher. The number is usually in your service agreement, but plenty of operators discover it the hard way — by watching rejections appear the moment a big campaign launches.

What happens when you exceed it

Push past the CPS cap and the carrier stops accepting the extra attempts. You'll typically see:

• 503 Service Unavailable responses — the carrier's way of saying it can't take the call right now.
• CONGESTION — fast-busy tones or calls that fail to set up at all.
• Wasted leads, because each rejected attempt may still count as a dial against a lead even though it never reached anyone.

flowchart TD
  A[VICIdial places a call] --> B{Attempts this second under CPS cap?}
  B -->|Yes| C[Carrier accepts call]
  C --> D[Call sets up normally]
  B -->|No, over cap| E[Carrier rejects attempt]
  E --> F[503 Service Unavailable]
  F --> G[Congestion, call fails]
  G --> H[Lead burned, no contact]

Where this fits

Knowing your CPS cap is half the battle — the other half is staying under it without starving your agents. For the practical pacing side, see throttling calls-per-second to protect your carrier. Spreading load across more than one provider also buys headroom — see running multiple carriers with failover. For the full carrier setup, see the VICIdial carrier integration guide.

VICIfast gives you a dedicated, fully tunable VICIdial box — provisioned in under 40 seconds — so you control the pacing that keeps you inside any carrier's CPS limit. See pricing to get one running.