About sip trunk register

hi, everyone

I’am very confusion about sip trunk register.

I have a voip account, and can use xlite register and call out or call in. if it mean I can use this voip account as my freepbx sip trunk?

Actually, I try to configure this voip account as my sip trunk, but it can’t register. why?

at same time, My another voip provider’s account can be registered as a sip trunk, and it does work.

Does anyone can tell me what’s different between xlite register and sip trunk register?