pinhole timing?

Hello All,
 
Can someone please tell me how long m0n0wall maintains a pinhole when an inside client makes a hole to reach an external host? 
 
I have a SIP phone that's losing access to the far-end SIP registry server. When the fault happens the external server tries to ring the phone but it generates a (SIP)408-Timeout. 


I've set the SIP keep-alive to 30 seconds in the phone, but that doesn't seem to have helped.
 
It's not clear if this is something new since I installed the latest release a few weeks ago.
 
When the 
 
Many thanks,
Michael