Hi ​@Emz Mac 

The "B" symbol next to the Wi-Fi icon on an iPhone 15 indicates that Wi-Fi Calling is active and the phone is connected to a Wi-Fi network for calls. Wi-Fi Calling allows you to make and receive calls over a Wi-Fi network.
If your iPhone 15 is displaying "P" with "No Service", it indicates a problem with the cellular connection. The "P" likely represents the Personal Hotspot feature, and the "No Service" message means the phone is not connecting to your carrier's network. 

Have you tried turning off WiFi Calling? 

Do you have the PH feature enabled? It may be worth turning this off to see if it affecting the network connection:-
To turn off the Personal Hotspot feature on your iPhone 15, navigate to Settings > Personal Hotspot and toggle the "Allow Others to Join" switch to the off position.
The iPhone 15's personal hotspot feature can, under certain circumstances, lead to a network connection being stopped or disconnected.


Have you enabled VoLTE?
https://www.eir.ie/helpandsupport/smartphonehelp/?page=device/apple/iphone-15/topic/mobile-network/how-to-enable-volte

Here is a website that gives advice with regards to this problem:-
https://drfone.wondershare.com/iphone-problems/iphone-15-pro-no-service.html


Hope you find an answer x