Fongo Mobile can not be used on an ATA. Fongo Home phone can not be used on any ATA other than the one sold (currently a Grandstream model) by Fongo. So, it's impossible to use Fongo with an Obihai product. Freephoneline and Fongo are sister companies owned by Fibernetics and do not offer the same services. It costs money ($5+tax) to port phone numbers between Freephoneline and Fongo Home Phone or Fongo Mobile. So, it's not a good idea to confuse these services. Fongo Mobile, Fongo Home Phone (not a BYOD service), and Freephoneline (BYOD service) are all separate services. Freephoneline and Fongo's servers have different IPs. You may see "powered by Fongo" on FPL's webpage, but Freephoneline and Fongo are treated as being sister companies that are both owned by Fibernetics.
VoIP.ms, Anveo, Freephoneline, Google Voice, etc., can be used with Obihai OBi200/202 ATAs, simultaneously, and all calls be routed between any of these services using Obihai ATAs. Further, a Google Voice phone number is not required to make outgoing calls with an Obihai ATA (just a Google Voice account is required).
Provided the phone number someone wants to use is already provisioned with a service on someone's Obihai ATA, nothing is stopping that person from dialing into it on their cellphone and placing an outbound call with that phone number using the service provider that provides the phone number. Nothing is stopping an incoming call to that number from be routed to that person's cellphone either. And no flaky cellular data signal is required.
Fongo Mobile is free with the exception of sending SMS (and receiving verification SMS from whatsapp, for example). Freephoneline is only free if you use its free desktop app (this is how Freephoneline got its name). Otherwise, Freephoneline's VoIP Unlock key costs a one time fee of $79.95+tax. Fongo Home Phone costs $4.95+tax per month. There's nothing free about Fongo Home phone nor Freephoneline's VoIP unlock key (intended for use with any SIP device/service).