Outbound calls no sound until call redialed

This issue is intermittent but seems to occur when the pbx has not been used for a period of time - say 30 minutes or more although this is not definitive.

The fault can best be described as…

An outbound call is initiated from A to B.
A picks up the phone (Polycom 601), hears a dial tone and dials the number. From this point, A does not hear any more sound.
Meanwhile B’s phone rings and they try and answer but “nobody is there”.
A hangs up and redials immediately. The call goes through perfectly.

System details:

FreePBX 5.211.65-4
Service Pack 1.0.0.0

Static IP

Details of setup are:

sip service is PennyTel.
Trunk Peer details (with changes to hide account details)…

disallow=all
allow=ulaw&alaw
canredirect=no
host=202.85.243.115
insecure=very
qualify=yes
secret=secret
type=peer
username=12345678
dtmfmode=rfc2833

No issues with inbound or internal calls.

Any help with this issue would be very much appreciated.