A listing of various SIP resources was published on Github at:
It focuses on open source variants, although there are several freeware and paid options, being build mainly from the perspective of Daniel-Constantin Mierla, one of the Kamailio co-founders.
Books about SIP, softphones or hardphones, SIP cli tools, network sniffing applications, native or java script libraries, SIP servers or PBXes, telephony operator platforms are among the categories included in that directory. Definitely some of them could be useful to use from time to time.
Thanks for flying Kamailio!