voicemail box retention after switching mobile carriers

So after 5 years and two iPhones I am finally leaving xfinity for t-mobile. I have contacted support twice this month, both times they say they will get back to me but they just immediately close the tickets. My voicemail is still the same Verizon visual voicemail from like 8 years ago but now I have an iPhone so it doesn't work correctly and nobody can fix it. So I need to leave and have my voicemail box purged from the Verizon backend before I come back. Does anyone know how long that will take to be purged? I will probably come back when there is some promo.