The phone needs to be SIM-compatible - have the same size slot for the SIM card. This shouldn't be an issue because Public Mobile uses "Triple Punch" SIM cards which can be resized up or down as needed, the plastic spacers around the SIM card are just plastic spacers, they contain no circuitry and they can be used with any SIM card from any provider, they can even be replaced with cardboard cutouts if you've lost the originals. Public Mobile does not (yet) support eSIM formats. The phone must have a physical slot for a physical SIM card.
The phone needs to be radio-compatible. All phones sold in Canada (and most phones sold in USA) are automatically radio-compatible with Canadian networks. But if you buy your phone from Alibaba or whatever then be careful to check it is compatible with the necessary protocols, frequencies, and bands. It might be a phone intended for foreign markets and regions which use different (and incompatible) radio standards.
The phone cannot be blacklisted (flagged as lost/stolen/etc on the databases). This won't be a problem when buying a new phone but be aware when buying a used one.
The phone cannot be locked (carrier-locked). Canadian carriers are not supposed to lock their phones anymore, it's technically "illegal", but in practice in the real world they do not always fully comply with this regulatory mandate.
Any compatible and "unlocked" phone should work after you put your Public Mobile SIM card into it and restart. You can freely transfer this SIM card between (compatible and unlocked) phones all you like, every day if you like, back and forth if you like, without any need to inform Public Mobile or to reconfigure anything on your Public Mobile account.
If the SIM card works in one phone but not another, then the problem is not the SIM card, it is the phone.
