Whilst this post is positioned for users of Draytek routers a popular product for business requiring a highly configurable powerful router to replace a standard offering you may hit issues with failing inbound calls after periods of inactivity on the handsets.
This is due to the SIP ALG, a SIP ALG is integrated into most modern routers and now comes turned on by default, This is essentially the router trying to be clever with your VoIP traffic rewriting IP addresses and ports within the SIP headers.
Simply if your having issues turn it off, this is either in the web interface in some models, Look for SIP ALG under Network->NAT->SIP ALG
If this cannot be found in the web interface you will have to utilise a telnet connection, telnet to the router, login with the web interface credentials
Please note the commands maybe slightly different from below, e.g. some models don’t require the commit
Issue the following commands:
sys sip_alg ?
Checks if SIP ALG is enabled or not, if it is:
sys sip_alg 0
Saves changes to the router and restarts the device.