[SR-Users] TLS connection with short lifetime - how to handle it?

Greetings. 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.