Unfortunately, 100% national indoor coverage is an unrealistic expectation - it's the nature of radio networks that good and bad coverage areas will always feature.
WiFi-calling itself is a very reliable and stable service, but is only as reliable as the underlying broadband & router. I don't know about EE broadband's free router, but a couple of other major ISPs let you use your own router with mesh and/or extension pods as required. It's what I use to extend wireless coverage where the freebie equipment just didn't cut it.
