If you are looking to use the number on your phone then you could install a SIP phone app and use your details in that (assuming you have bought the unlock key).
If you no longer want the FPL account and just want to have the number on your phone, then you could port your number over to Fongo mobile.
https://support.fongo.com/hc/en-us/articles/212436246This would close your FPL account and you would forfeit the unlock key if you have purchased it.