The answer to the titular question is, "Yes, and it need not even be a *treasonous* phone call."

[In the United States anyone can be justified in using lethal force](http://law.stackexchange.com/a/14974/10) if they can establish that, in the moment, it was *reasonably necessary* to prevent *imminent*, *grievous*, and *unlawful* bodily harm to themselves or to another person.  (The italicized terms may vary by jurisdiction, but AFAIK the same idea and meaning are codified everywhere.)

How can making a phone call cause imminent death to another?  Examples that come to mind: One could be keying or relaying the code or command to detonate a bomb.  One could be confirming the name of an undercover agent who will be summarily executed if outed.

In the particular case described in the question, one *would* expect the soldiers to shoot if commanded.  When military enlistees are handed a gun and given an order to shoot, they are expected to not put a great deal of thought into whether to follow that order.  Unless it is a *clearly unlawful* order, that is, one that they know violates military law, standing orders or rules of engagement, they will be disciplined for *not* shooting on command.  (It is the commander who *might* face a more detailed trial at some later date into whether the order was lawful given the circumstances.)