@bigsy It sounds fairly immune to that scenario then. What I notice here, with my n300 is, that after SIP registration, the only SIP traffic from the N300 are SIP OK responses to SIP OPTIONS from the server (local asterisk). This I took to be a keep-alive mechanism.
When the firewall state's lost for whatever reason, some form of SIP packet is needed from N300 to recreate the state. That didn't come until the next SIP REGISTER and my unit's was set at 3600.
I have pf states set to conservative, which AIUI keeps UDP states for 900s.
Having said all that, N300 and two handsets have worked quite well for me. Android softphones are another matter!
Thanks for the discussion.