How do I limit maximum call duration on v17

FreePBX Configuration
4

Don’t use from-pstn-custom. You will find references here in the forum to its existence, but it should not be used. A supportable way of adding your own dialplan on the inbound leg of a call is to create your own context that looks something like this

[form-pstn-preprocess]
exten => _.,1,Noop()
exten => _.,n,Set(TIMEOUT(absolute)=900)
exten => _.,n,GoTo(from-pstn,${EXTEN},1)

then set the context of your trunk(s) to the above context instead of from-pstn.

For outbound calls use the predial hook explained in this thread