Don’t thinks the PBX for VoIP calls issue are fully descried in the earlier post.
UDP NAT timeout it’s more to the standard value defined for NAT device:
UDP session 30 seconds
UDP established/streaming session 180 seconds
UDP SIP 5060 session 3600 seconds.
The timeout value will keep refreshed back to the default value when traffics is sent via the sessions.Sessions will only being drop/remove when no activity for the session after the timeout value and normally this will happen if the sessions is no longer being used.
Regarding to the VOIP phone issue, i strongly believe the issue is more to the PBX/SIP/VoiP phone configuration issue whereby most of the system should defined with NAT keepalive to refresh the standard session timeouts more than changing the NAT timeout for the NAT devices. You can find some of the related info using the URL below: