I also have been having this problem but didn't realise as the line was making mainly outgoing calls. I believe the problem may be associated with keeping the SIP session alive with the server. It only seems to happen with 3G as I've not noticed this on my other VoIP line running over ADSL.
I don't know how long it is, but when I call the 3G line after it's been up for a while it goes straight to voicemail (meaning my call isn't connecting to the SIP client). If I make a call from the 3G phone, then I can receive incoming calls to it again. I'm guessing that making the call causes the router to re-register with the server on that SIP line, so it'll be able to receive calls again until whenever it is that the session gets dropped again.
I'll look into this again when I have time, but my guess is I'll need a keep-a-live tweak or maybe an outgoing proxy configured. I'm open to suggestions however as I'm no VoIP geek.