Greetings craigfrommanchester,
Thanks for visiting the Apple Support Communities! I understand you and the other party aren't able to hear each other while on a call. Further, you mentioned this behavior persists with two different devices you own, and only occurs when the other party is using a mobile device.
To help troubleshoot this behavior, I suggest to restart your iPhone and check for a carrier settings update. You can see those steps from the following support articles: Restart your iPhone, iPad, or iPod touch & Update your carrier settings on your iPhone or iPad
If the behavior continues, it would then be best to contact your wireless carrier for additional assistance.
Let me know how it goes from here.
Kind regards.