How to register an IP phone or softphone to S-Series VoIP PBX?

S-Series VoIP PBX have passed compatibility test with lots of IP phone brands, such as Yealink, Grandstream, Cisco, Polycom, Snom, etc. Any questions about the Yeastar S-Series VoIP PBX, please refer to https://www.yeastar.com/s-series-faq/.