@John_P PM won't magically change the phone number. You sure you didn't manually change it?
another reason that happen is a failed porting request
did you join PM last month with a porting request? if you did, likely you didn't reply YES to a text from previous provider and it was on hold for a month, and expired
To get it back, check with your old provider if the acocunt i is still active. if it is, you can re-request porting once more. Let us know and we will advise further
@John_P It’s tuff to say could be gone for good , but you ask support
Add you can change your phone number yourself in my account to a new number 4x each 30 day cycle for free fyi