As noted by jimates, the OBi cannot detect fax tone.
If most of your faxes come from dedicated numbers, you could set up the OBi to reject calls from those numbers; FPL voicemail would answer them and receive the faxes.
Although you would still be disturbed by the calls, you can set your fax machine to automatically answer when it hears fax tone, then simply hang up the phone when you hear the machine pick up. If you did not answer, FPL voicemail would receive the fax.
If a separate incoming number for fax-to-email meets your needs, possibilities include a free number from
k7.net (it would be a US number in Washington state) or an Anveo Value number in your province ($1.45/mo. including 40 min./day free incoming).
You could route incoming calls from FPL through another service such as VoxOx, which would intercept faxes and pass other calls to your OBi, but that would add latency and degrade voice quality and reliability.