ETA: Because context matters, the background here is that faculty member X dropped in to ask about a problem he was having with software product Y. The person he wanted to talk to was in a meeting, so I took a message. end of added context
I went with, "Could you call X about Y, please?"
Here's how I reasoned it.
"Could" rather than "Would" -- "Would you call" sounded, in my head, like an instruction to do so, whereas "Could" had, for me, the connotation of a polite request that understood the co-worker might not have the time to make another phone call.
"call..., please" rather than "please call" -- the former, again, sounded in my head like a polite request, where the latter sounded to me as if I were exasperated because I'd made the same request several times before.
Huh. I just realized that if the person who needed to be called had been of a higher perceived status than my coworker or had conveyed a sense of urgency, I probably would have written, "Please call X about Y".
For the future, I may avoid the whole thing by moving to something like, "X called with a question about Y. His/her number is Z. Thanks!" and simply assuming the contact would be made.