Cannot trasnfer call - intermittent

A couple times a day we take an inbound call, put it on hold and can’t transfer it to another extension after we take it off hold.

The system is Asterisk the phones are Grandstream GXP20xx. It only happens about once in 20 calls but it’s very frustrating to the users. It’s a blind transfer and when you press the transfer button it beeps instead of coming back to say “transfer”.

What can we do to debug and fix it.

There is nothing unusual in the logs.

sip set debug on

In particular look for the INVITE with the


line, and what follows it.

Thanks David,

Where does the SIP debug info get written? Will it end up in /var/log/asterisk/full?