Please visit
https://support.freephoneline.ca/hc/en- ... Follow-Me-, and read the section on "Limitations/Restrictions".
If no restriction listed applies to you (forwarding FPL to FPL, for example), submit a ticket:
https://support.fongo.com/hc/en-us/requests/newChoose "My Account Inquiry" for the final issue type.
Users and moderators can't resolve this for you.
How to find out what it's used?
You can't if the number does not appear in any of your Fongo or FPL accounts.
Or can be removed from the currently used?
You can't unless the number appears in any of your Fongo or FPL accounts (in the Follow Me section).
Otherwise, only support staff can.