Some VoIP providers sell SIP with limits like POTS because people are stupid and have no idea what they are buying.
My god it's a pain to find this i'm struging to find a supplier in the UK that says 1 SIP trunk allow x calls lots say 1 trunk = 1 call
At least this one shows 2 in 100 out
Good god man use twilio elastic trunks and a [email protected] virty. It'll be fine. ️