I have registered my account with freshphone, and I am currently using it with the Zoiper Android app.
I have received 2 calls, that worked perfectly, but a subsequent 2 calls where the caller could not hear me besides a very mumbled "gurgle" as it was described.
All 4 calls were minutes apart, and all while the handset was connected to my 100mb fiber line.
The phone is a Samsung S7 edge if that helps.
Was the issue fixed when you used your mobile data?