I set up servers on the net that masquerade as a SIP relay by essentially supporting the protocol but with few authentication protections. Malware bots scan the IPv4 space looking for such machines that they can use as a relay. My honeypot is actually an extensible framework, and we also can mimic SSH, Telnet, HTTP, SMB, FTP, RDP, SMTP, MQTT, Node-Red, MODB, S7, and SNMP. Individual servers can easily be set up to scan any subset of those. Check out https://knock-knock.net to get a visual sense of what the honeypot is doing!