I got a call after an agent read my partial, and we discussed a bit about my career and where I saw myself as a writer, etc.
He still wanted to read the full before offering me representation (and did, two days later).
All the rest of the time? Emailed, or a written request for more (actually, 90% of the requests for more came via email).
So it DOES happen every now and then, but I wouldn't sleep with your cellphone next to your ear yet.