Can you expand on what you mean by "very intermittent signal"?
I only ask because... if your partner's phone is with EE and has no service, then the consequences for calls, texts & mobile data are predictable - i.e. an inability to use those services.
But if your phone is getting some coverage, that seems to contradict the "no service" scenario. The logical conclusion would be that there is very poor coverage (for whatever reason) and your phone has fractionally better radio performance.
If you're in the UK and have a WiFi connection, you can use WiFi-calling to keep using your phone as normal without needing network coverage.
