When you say "breaking up", do you mean dalek-type speech in one direction? If so, I've got a personal theory on what may be causing it, but that's based purely on observing patterns and having some network-knowledge.
When you refer to WiFi-calls, many people take this to mean audio-sessions over apps such as WhatsApp, Messenger or suchlike. In technical terms, WiFi-calling means the network-based facility where you still dial & receive calls as if they were over the mobile network, but you're using WiFi as the connection to EE's central network instead. Which are you referring to?
In the first instance, see if you can establish whether this happens in one specific location or if it's more general. The online status tool - https://ee.co.uk/help/mobile-coverage-checker > check service status, has an option to report problems with in-call audio. That's the first stage, if issues persist then the problem may lie elsewhere and you should start collating timed & dated examples to report to CS.
