How to get off cell phone plan

My son is current a member of our family cell phone plan but is trying to get off our plan and start in his own plan with Tmobile. He has tried calling tmobile months ago and changing cell phone plans but they said they were doing a credit score check and that they would call back but they never called back. Since then we have not gotten on the phone on them for longer than a second once but when we did get on the phone with them they said we can do this online. Can you please tell us the way we can do it online? Every time we have tried calling no one picks up.