Hi mr.pavel welcome to global voipforum .  Do you have solution for  push notification forward using kamailio  for asterisk or freeswitch sip users with  nonblocking  asynchronous operation .
Kamailio is excellent choice for call routing with its high performance structure . Pbx/softswitch providers  combine kamailio with media aware platforms like asterisk & freeswitch and offering solutions .  Please let me know if you have solution (kamailio config) for registered sip clients on asterisk&freeswitch .