My son has a Pixel 6a with iD Mobile SIM, and hasn’t noticed drops in network performance. 

Is this a call/texts issue, or a mobile data issue? 
Does this problem occur everywhere you use your new Pixel 6a device? 

It's mostly affecting calls/texts. And it doesn't happen everywhere, but it happens in places I got perfect signal in my previous phone (OnePlus 6t) with the same SIM. Is there a setting I can change or something do you know?

It also shows on the device that I have a decent signal, yet calls just fail/break up too much to have a conversation 

It’s possible network congestion could be affecting your voice calls @cdwhitehead. 

Three UK (the iD network partner) are decommissioning their 3G mobile network by end of 2024, which means all voice calls on iD Mobile will need to use 4G Calling (VoLTE). The OnePlus 6T isn’t listed by iD as supporting 4G Calling.

3G has been switched off in some areas already, but even if 3G is available I think iD will prefer 4G Calling on devices that support VoLTE. This can cause issues if there’s network congestion, or limited capacity on the backhaul network. 

Have you checked the current network status in your area/s @cdwhitehead? 
Check online at https://www.three.co.uk/support/network_and_coverage/network_support.

At home it says good signal, but I still get poor quality calls. At work it says no signal so that explains that! It's just confusing as I swear I had perfectly clear calls at home on my old phone 

Your old OnePlus could probably only make 3G calls @cdwhitehead, and wasn’t capable of 4G Calling like your new Pixel device.

Perhaps, you could force your Pixel 6a to use 3G mode, which might improve the call quality. 

Hi @cdwhitehead sorry to hear of the trouble you have had with coverage. Have you tried switching between 3G and 4G? Do you find one better than the other?

Thanks