What might cause a phone to only answer on the second attempt?

Commercial Products and Partners Digium
1

Have some Digium desk phones - one of them has just started only allowing calls to be answered on the second attempt - any thoughts on what might cause this? Probably not without logs. I’m not near the server now, but when I am next, I’ll do a complete call trace with sip debugging on, if that is going to be more useful!

thanks

2

Are the phone and FreePBX server on the same network?
If not, and if NAT is involved, probably the session is expiring and the first call reconnects the session, then the second call comes through.