Re: So if
"One the assistance needs to be listening into the call to hear the trigger phrase. The second and far bigger problem is what should be the end phrase."
Neither of these are problems at all. The former is no different from the mere existence of voice assistants. If you don't have a problem with it listening to you at all times waiting for an activation phrase, why would you suddenly have a problem with it continuing to function during a phone call? Most people are perfectly happy to use their phone while sitting next to an Alexa speaker, and this is no different.
As for the second, far from being a bigger problem we've already demonstrated that it is not a problem in any way. It was a running joke in the early days of voice activated thingies about how they would handle things like TV shows trying to be realistic by incorporating voice commands used in-show, or what would happen if two people in the street happened to be trying to use commands at the same time. The solution turned out to be that it's just such a vanishingly rare problem that no solution is needed. It's trivial to have an activation phrase that just doesn't come up in normal conversation, and even without having any filtering or voice recognition phones just don't pick up commands from nearby people - the idea of walking down a busy street and saying "Hey Google, call Mum" in a loud voice doesn't actually result in anything happening.
In this specific case, all you need is to say something like "Siri, end call". No-one is going to accidentally end a call unless they're deliberately screwing around.