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 .