I'm guessing because they want the identities of their recruits to be secret, or at least difficult for other intelligence agencies to determine. Calling from a cell phone makes a very clear link to a specific person, while a pay phone would require other methods of intelligence gathering to unmask the identities of potential new spies. It is probably easy for a nation state level adversary to hack, or otherwise obtain phone records (especially if the call is somehow routed through the US).
Comments
I'm guessing because they want the identities of their recruits to be secret, or at least difficult for other intelligence agencies to determine. Calling from a cell phone makes a very clear link to a specific person, while a pay phone would require other methods of intelligence gathering to unmask the identities of potential new spies. It is probably easy for a nation state level adversary to hack, or otherwise obtain phone records (especially if the call is somehow routed through the US).