Hello,
Unless your Android smartphone is connected to Wi-Fi, Fongo Mobile uses cellular data.
Fongo Mobile is a VoIP service and requires internet access to make calls.
It seems likely that you're being charged for cellular data usage through your cellular service provider.
To use Fongo Mobile for free, you need free Wi-Fi internet access/service.
You can reduce the amount of data that's being used during the call, but the trade-off is inferior sound quality:
https://support.fongo.com/hc/en-us/arti ... obile-use-.
Given that we're on Fongo's forums, I am unwilling to discuss competing services, but you are free to pursue them.
Fongo's solution would be to use Fongo Wireless:
https://www.fongo.com/services/fongo-wi ... -it-works/.
I've never used it, so I can't advise with respect to how well the service works (I respect seniors, and I don't wish to act as a shill by praising a service I've never used), but if you visit the link, you can read about it to see whether that service interests you.
Possibly other forum members have insights.
The Fongo Wireless forum is located at
viewforum.php?f=76 if you wish to read other threads.
Good luck!