If you use the modem and the manufacturer is saying that it's not fully compatible with Win7 then you're unfortunately going to have to live with the prompts. The UAC control doesn't have a whitelist unfortunately but there are a few ways around it (note, this is for Vista but I have to "assume" it will work with Win7)..
http://www.beingmanan.com/wp/2008/06/create-uac-white-list/
Cheers,
Brian