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).