Porting an old landline number to a budget prepaid SIM for an elderly parent?

Hi everyone. I’m trying to move my mother’s old copper landline number over to a cheap, prepaid cellular plan so she can keep the phone number she has had since 1985. I bought a basic flip phone for her to use at her assisted living facility. Has anyone successfully ported a traditional wireline number to a prepaid MVNO network? I've heard the transfer process can take weeks and sometimes fails completely if the account details don't match perfectly.

Expect the transition to take at least five business days. When I moved my grandmother's copper line, she was completely without incoming calls for almost a week while the two telecom networks communicated. Definitely do not cancel the original landline service until the new cell phone starts ringing successfully.