I really don't see that happening any time soon. It'd have to query the GAL over the internet prior to accepting the call causing delays in call handling. I can just foresee dropped calls and what not as a result. Also, in Verizon's case, unless the iPhone hardware changes or they switch to VoLTE in all areas, no simultaneous Voice & Data.
Otherwise to circumvent this, it would have to store your GAL locally. With a few hundred people isn't much, but there are some organizations with 100K+ employees. Seems like a bunch of useless data to store locally when you probably deal with 1-10% of the users, especially if you want to include contact pictures, etc. I don't know of any mobile device (including Windows Phone) that would do this today.
Probably the best thing is you'd be looking at doing something specific with your company's phone system (aka Microsoft's Lync) that would do that all in line with the call handling.
This is mostly just my opinion, dealing with this type of tech all day.