Why can't I hear anyone when I make a phonecall?

[AC Question]

When I make or receive a call on my Wileyfox Swift, I can no longer hear the other person (or they me). An icon - a solid circle with a minus in it - appears in the status bar. I assume I have turned something on or off accidentally. Other sounds are working fine, I have turned up all the sounds, can hear the radio etc. I have looked at every setting i can find to no avail, can anyone help me turn it back to normal?

 

[GameDus IO]

That happens to me every other day and in certain parts of the city, no matter if it's a local or international call. Sadly I found that's an on going issue with my current network company, they have grown so much and the users are paying the price.