ok,
A possible reason is, the network resources.
Here is a quote from some other forum post:
http://forum.xda-developers.com/viewtopic.php?t=51814
"GPRS can use 1, 2 or 4 data slots depending how busy the cells (towers) are in your area. If you happen to be using 4 slots and you get a call the call will not get through, if you're only using 1 or 2 slots then your call can get through... "else,
May be while downloading the file, more slots are getting used?
Are you sure, while browsing (during download of the page), are you able to get the
incoming call notification?