RFC 3261
OPTIONS keepalive
OPTIONS asks a target if it is alive. It is not a call, and a timeout can take the target out of service.
- OPTIONS — Sent on an interval you configured, often per dial-peer or server-group.
- 200 — The target stays in service.
- OPTIONS — The next probe, after the interval.
- timeout — Repeated failure busies the target. New calls hunt to the next preference.
If half your calls use the expensive carrier, check preference and whether keepalive has busied out the cheap one. Do not 'fix' it by adding a second identical destination-pattern at the same preference.