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 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.)

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 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.)

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 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, 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.)

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 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.)