[Windows Central Question]

Hi all! I purchased a Xiaomi Mi 4 via eBay that had the Windows 10 ROM already flashed on it. I am trying to use it on AT&T in the U.S. Everything is working great, except for voicemail. AT&T voicemail service has difficulty recognizing my mailbox number. I know the voice mailbox number is correct as I have swapped the SIM into a Lumia 640, which resets the voicemail box at start, then works again. When I swap the SIM back into the Mi 4, voicemail (including visual voicemail) will temporarily work. But then AT&T's voicemail service will ask for a voice mailbox number, where inputting any number (including my number and the default voice mailbox number) will not work! I have called AT&T customer support, but with no help. I previously concluded that this may due to the lack of proper LTE bands, but why would the voicemail box work, then suddenly not work? As a work around , I am using the 3rd party YouMail app, ISeeVM, but would really like to start using the built-in voicemail. This question also pertains to other AT&T users, with unlocked, international phones. Thanks for your help!