[Cypher]

I'm not sure why this would be a problem as you could easily implement it using hash tags or something similar to get around it and then search on the tag.

Here's an example tasks with three embedded hash tags for your three contexts.

Task: Talk to #Bob about a new proposal. #phone #client-xyz

Also maybe setting a base Context of Multiple would provide a way of also finding tasks that had multiple context.
Context: "Multiple"

As there would only be one task as such, marking it as complete will close it off from all three hash searches.

I've just done a quick test of this by setting my normal phone context to #phone. Then in some other tasks, which weren't already phone tasks, I just added the hash tag #phone to the end of the existing task description.

Searching for #phone now quickly shows me a list of all my "phone" tasks and includes all those contains #phone in the description or in the context.