Hello Vallimamod,
The best way to keep a user behind nat, is sending OPTIONS:
modparam("nathelper", "natping_interval", 30)
modparam("nathelper", "ping_nated_only", 0)
modparam("nathelper", "sipping_bflag", 7)
modparam("nathelper", "sipping_from", "sip:pinger at PROXY_IP")