People can't hear me when I call them?

My iPhone 5s more often than not fails on first attempt at a call to someone.

Here’s what happens.

I call someone,they answer,I can hear them but they can’t hear me. I hang up and re dial and it’s fine. It doesn't matter whether I'm on the handset, loudspeaker or my BT earphones with mic, it happens with them all.

This started happening around 10months ago. The handset is 25months old.

I’ve tried lots of things,turning off WiFi,resetting phone to factory settings and setting up as new phone., latest iOS...nothing helps. I've been through various iOS updates so don't think it's software.

My carrier sent me out a new SIM card to try and resolve but it's not helped at all.

I have decent signal most of the time when it happens.

Anyone able to help try and pin point issue?

Thinking it could be a hardware issue?