Hi @Natasha Persad 

As your device is on the Approved Handset list with iD Mobile you should be able to connect to the VoLTE (4G) network if the 3G signal has recently been turned off in your area.
Have you checked the network coverage in your area to see if this is the case?   https://www.idmobile.co.uk/help-and-advice/coverage
If you can still send/receive texts and use data then the turning off of the 3G network in your area is most likely the cause of your loss of calls.
Do you have the VoLTE toggle and has it been enabled on your device (Settings/Connection/Mobile networks)? https://www.sunrise.ch/en/support/mobile/mobile-phone-instructions?page=samsung-galaxy-z-flip-5-android-13/call-settings/activate-volte/p5_d26016_t17917_o1305

If you can use WiFi Calling then VoLTE should also be provisioned on your phone.

Thanks for the help @Decembersangel72 - @Natasha Persad did this help? Thanks.