Then you have a SIP trunk provider that doesn't validate domain ownership... I'd like to know that companies name if you don't mind sharing. They're stupidly rare to the point that I view it as a unicorn situation.
Edit: To clarify, I've tried finding such providers and failed for several years... They all want PTR validation for "security"
Your local college might do networking courses/stuff. honestly though, there's enough youtube content out there by really respected people that you can likely just get away with that... Start with words/topics you see mentioned in this thread. Example, search youtube for DDNS... and if that video says something you don't understand search for that topic. Eventually you'll have a decent grasp on what's going on.