What VICIdial Call Progress Detection Is vs AMD

People use AMD and CPD as if they mean the same thing, but they do not. Answering machine detection (AMD (answering machine detection)) answers one question: is this a human or a machine. Call Progress Detection (CPD (call progress detection)) answers a broader one: what exactly is on the other end of this call. That wider classification is the whole reason CPD existed as a separate, paid engine.

What CPD classifies

CPD is a call progress detection engine that listens to the early part of a call and sorts it into categories Asterisk AMD cannot reach on its own. Beyond human and machine, it can identify fax machines, SIT tones, and more detailed groupings of disconnects and congestion. SIT tones are the three rising notes a network plays before a recorded error like number not in service, and being able to read them gives you a much more accurate picture of your leads.

The best-known engine here was the Sangoma NetBorder Call Analyzer, sometimes referred to as ParaXip CPD. It was a proprietary SIP gateway that claimed very high accuracy in fast human-versus-machine decisions and added pre-answer classification on top. It ran as a SIP proxy, worked best on a separate machine from the dialer, and was sized at around 200 concurrent outbound calls per quad-core box. Sangoma set it end-of-life in 2017, so it is no longer sold, but the concept and the campaign settings around it still exist.

How CPD differs from AMD

The built-in AMD is the Asterisk AMD application running inside your dialplan (Dialplan). It is free, it ships with VICIdial, and it is good enough for many campaigns. But it only judges human versus machine and it cannot read SIT tones or finely classify disconnects. CPD was a separate licensed product that traded cost and extra setup for that richer detection and higher accuracy.

flowchart LR
  A[Outbound call answered] --> B{Detection engine}
  B -->|Built in AMD| C[Human or machine only]
  B -->|CPD| D[Human]
  B -->|CPD| E[Machine]
  B -->|CPD| F[Fax]
  B -->|CPD| G[SIT tones]
  B -->|CPD| H[Disconnect or congestion]

Why the extra detail helped

There was a real metric benefit. By identifying calls that were never going to be a live person, CPD let you exclude them from your answered totals, which lowered your dropped-over-answered percentage. For campaigns watching a drop rate (Drop rate) against safe-harbor limits, that cleaner denominator mattered. It also meant fewer mystery dispositions, because a SIT tone got labeled as a SIT tone instead of guesswork.

Since the Sangoma engine is gone, KHOMP offers a SIP gateway for AMD and CPD as an alternative, with its own settings container, admin tool, and quick-stats report. But for most operators today, well-tuned built-in AMD plus a sensible dial timeout (Dial timeout) covers the need without a separate license. The dial timeout trick is simple: most answering machines pick up in a predictable window, so capping ring time at 22 to 26 seconds dodges many of them with no detection at all.

So the short version is this. AMD is the free, built-in human-or-machine check. CPD was a broader, paid engine that also read fax, SIT tones, and disconnect detail, and it could tidy up your drop percentage in the process. They solve overlapping problems, but CPD aimed at the harder edge cases that the basic detector simply cannot classify.

To see the setting that activated CPD on a campaign, read the CPD AMD Action setting, and for the full landscape see our AMD and CPD complete guide.

Want detection that fits your traffic without buying a separate engine? We tune AMD and advise on CPD options as part of managed hosting. See VICIfast pricing.