I just want to finish my device payment

I switched providers since I don't have the internet with xfinity and i got tired of paying extra because of it. Now yall won't let me make my final device payment. I've tried calling they tell me to go online I go online they disconnect or I can't reach anyone. There was $333.43 to unlock my phone and it wouldn't let me do the $0.43 just the $333 and now that thats processed it wont let me do the remander balance to unlock my phone for my new service provider. Xfinity is holding my phone hostage at this point.