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...
Hi Guys
About a week ago our telephone line went simply dead, no dialtone at all and nobody can phone us but at the same time our ADSL on the same line works 100%.
Logged a voice fault and must unfortunatelly admit that a Telkom tech arrived within a few hours. Did NOTHING here but said...