Distorted audio on recipient’s end when calling them (I can hear them, they struggle to hear me)

I have an odd question for you that I can’t find the answer to anywhere.

When I call mobile phones abroad (from the UK), I have been told that there are difficulties hearing me and that the call ’breaks up’ or that the speech ’sounds bubbly’. I can usually hear the other person well, but occasionally - more often recently - the line goes completely quiet for short moments (but they can still hear me!). These problems first started occurring about six months after I bought a new phone, an iPhone XS, so I thought maybe the microphone had become dirty or obstructed in some way. However, carefully cleaning it has not made any difference. I have also tested the microphone by recording my voice, and the playback sounds as it should.

What I find weird is that there are no issues when I call mobile phones in the UK, nor when I call ’home phones’ abroad. The problem only seems to be with mobile phones abroad. I have not been able to test a different SIM or network to see if that works better.

Can this be a hardware (or software?) issue with the phone, or is it a connectivity issue with my mobile network provider?

Thanks for your help!