How to connect a FRITZ!Box as a client to an asterisk 1.15.x+ with pjsip

How to enable internal text communication using ip-telephones connected with pjsip to an asterisk 15.x+ to send and receive SMS locally.

How to connect ip-telephones with pjsip to an asterisk 15.x+

How to connect an asterisk 1.15.x+ with pjsip to a Telekom all-ip connection (tel.t-online.de, t-ipnet.de)