No sound during calls (intermittent)

I often find that when I receive calls I can’t hear the caller, though they can hear me. There is no sound from the phone at all. This problem is intermittent but is quite common. I don’t think it happens when I make the call. I have two SIMs on the same network (GiffGaff) and it occurs when receiving calls on both numbers. I have an FP1(U).
Any ideas what might be happening? Many thanks.

Does that by chance happen when the person who calls you uses an FP2?

No, I get it when my girlfriend calls from her iPhone and from customers where obviously I don’t know their device but I assume very few have FPs.