Guest Wifi - What can our Marketing dept. capture?

Hello!

I have a marketing department that wants to be able to capture everything about a guest device on our wifi (even the phone number). I told them I'm pretty sure that's illegal. The best I can do are the MACs. Does anyone know of any website(s) that I can direct them to as far as what we're allowed to capture? I am located in Canada....but anything would help. TYIA!