As far as I was aware on the Neotel network your internet (data transfer) would stop completely when making/recieving a voice call. AFAIK this issue was brought up in a number of threads about Neotel's service etc. (can't be bothered to find one right now though.)
In this thread, http://mybroadband.co.za/vb/showthread.php?t=134728, PeteGT mentions that his data transfer rates drop to ~5KBps when using voice on his Neotel, which is in opposition to what I previously understood, i.e. that data stops totally.
So I just want clarification from actual users on what does happen in their case (not knowing if this is normal or not), maybe some input from Neotel also?