My dad got a new XS Max in May but was having the problem of calls dropping intermittently, I upgraded my phone in December so gave him my upgrade of an iPhone 11 Pro Max and is still having the same issue. Happens both on WiFi and 4G, do not disturb is turned off so that is not the issue and not just in one location so wondering what next steps could be?
@amystewart Try a network reset on the device, go to settings>general>reset>reset network settings. The device will restart and you will need to put your WiFi passwords back on the device. It might also be worth requesting a replacement SIM card from EE, going to an store is the quickest way and you’ll not pay for postage.