Poor call quality

[tricolas]

I've recently switched to sim only from my contract and the call quality is really poor, i.e. people can't hear me when I'm speaking to them. This is a recent issue, so I'm wondering if the move from contract to sim only has somehow done something, even though it's the same sim card.

My wife is on Tesco, which I believe uses the o2 network and she rarely has problems in the same areas of the house. 

Is it likely to be the sim card or phone (S9) which is causing the issue?

TIA 

[MI5]

See if you can try your sim in your wifes phone or vice versa @tricolas 

That will prove if sim or phone fault.

[tricolas]

Just tried her sim in my phone and it was absolutely fine.

Do you think a new sim card or moving over to tesco would be best?

[MI5]

I'd try a new sim first @tricolas 

Any O2 store will give you a new sim for free.