International VoIP providers, DTMF, broken SIP implementations (and more!)
(I started writing this at the beginning of May 2020, and probably finalizing near July; so yeah be warned.)
Ever since I released my writeup about the (in)security of Türk Telekom's home gateway management, the VoIP config I found stuck in my head.
This led to many attempts to create a large home phone system. I failed hard. However in the process of doing that, I learned how cursed SIP services are and even got a free Cisco IP Phone from an ISP contact.
SIP Trunking not working with...correct data?
SIP servers/gateways never completely follow the actual standards guiding the protocol.