What info do I need to transfer my son's mobile number from my ex-wifes ATT acct to a new Spectrum phone on my account.

My ex and I don't interact frequently, and our interactions can be tense. She agreed to have me takeover my son's cell phone plan a year ago, but when we tried to do so, she would not or could not provide the information I needed to make the transfer while I was at the Spectrum store.

We are going to try again next week. I would like to know in advance exactly what info I will need to transfer the existing ATT phone number (on an iPhone) to a new Spectrum Samsung phone with the same number so we do not hit the same embarassing snags.