I'm not sure I understand this question.
Why would FPL cost you per minutes fees if the call is within the free calling area?
I thought the idea is
A. FPL+PBX.
So provided the call from your FPL number to your cell phone is within FPL's free calling area, and provided you're within your cell plan's free calling area, you're only going to charged for cellular airtime (incoming minutes).
Or
B. VoIP.ms. No way around not paying fees with them. Both the incoming call to the VoIP.ms DID and outgoing from VoIP.ms to your cellphone will be charged. The call is bridged (in both A+B scenarios). But, as csi123 mentioned, VoIP.ms does have plans with included minutes.
I'm not sure I understand why you'd be using a PBX with VoIP.ms.