I had the same problem. First person I spoke with said I couldn't unlock it even though it was my own personal phone. I asked for a manager, waited several minutes and the original guy wanted additional information. I restated my wish to unlock my own phone that was previously unlocked. He then redirected me to another dept and they unlocked it within minutes.
By the way, when I went into an Xfinity office about unlocking my phone, the guy said Xfinity does not lock your phone which was incorrect in my case.
Took me close to a month to get my daughter's iPhone unlocked from Xtrash, constant lies from this company. Day that it was unlocked I was off to a Apple store (100 miles away) to get her an unlocked iPhone from Apple, trade in the Xtrash iPhone and get their service out of my house. There are some serious issues with this division of Xfinity, Internet is no problem but something stinks in the mobile department.