I have the next problem:

iOS based clients connect via TLS to kamailio server.

They run mostly in background mode - it means connection refresh interval is ~10 minutes.

Some of clients reside behind paranoidal routers which considers such idle connections as lost and closes them. 

I see only way to resolve it is to send OPTIONS/NOTIFY/CRLF from kamailio to clients.

Tried to use nat_traversal module by calling nat_keepalive() to every REGISTER message. But there is no incoming keepalive (OPTIONS or NOTIFY) on client side.

Is there another way to make "heartbeat" in SIP/TLS from server side?

Thank you.