A polite way to do it would be for the assistant to say: "Hello, my name is Bob. I'm calling on behalf of Alice who would like to speak with you on the phone.

A polite way to do it would be for the assistant to say: "Hello, my name is Bob. I'm calling on behalf of Alice who would like to speak with you on the phone. Might you have a moment to speak with Alice now or may we schedule a phone appointment for later today?" Of course, if the matter is not urgent but can't be handled by email, an even better solution is for Alice to send email requesting a phone appointment in the first place.