Originally Posted by
watson10
My hope was I could turn off the UK sim to avoid charges and turn on the esim for data and still use WhatsApp and iMessage normally. It sounds like isn't possible.
I use my domestic sim set to WiFi only which uses the data of the local sim to check iMessages without any roaming charges. The UK provider has to support WiFi calling with no geo restrictions for your destination.
This feature is often called IMS and has been discussed in this thread and a couple of other threads.