Correct me if my conversion math is off, it's been awhile:
Assuming 20 kbits/sec of data usage, you're looking at around 120 kbytes per minute each way. So 240 kbytes of data usage per minute when a call is being made.
Assuming a 1 hour call: ~14 mb
Online bandwith calculator says a 1 hour data call over GSM is about 15.56 mb.
Source: FaktorTel - VOIP Phone Company - Leading Australian VOIP Service Provider for Residential VOIP and Business VOIP
Not bad for an hour, but making copious amounts of VOIP calls could definitely push someone who was already a heavy user toward their data cap. I believe most "unlimited" plans cease to be "unlimited" when one hits 5 gig.