AB means Busy (carrier): the carrier returned a busy signal, with no agent involved.
A few things worth knowing about it:
- It is distinct from the agent-marked busy code. VICIdial files a carrier-received busy as AB (Busy Auto), separate from the agent-defined B that an agent sets by hand, and that distinction matters: AB is the dialer believing the carrier, while B is a human judgement call.
- AB specifically reflects Cause 17 in the disconnect detail: Cause 17 is user busy, meaning the destination phone was on another call and returned a busy signal.
- A busy signal actually tells you something useful. A busy is a kind of hangup cause, and unlike a reject or a timeout it tells you the line is alive, someone is on it right now.
If you're seeing a lot of AB, it usually isn't a dialer fault. A high Cause 17 rate is a pattern, and the pattern almost always traces back to one of two things: redialing the same numbers too fast, or a list of numbers that genuinely are in use a lot. In fact, AB is a healthy code in moderation, it proves the number is real and the carrier is honest, and chasing it to zero is the wrong goal.