You do NOT need to reflash; but you will need to do a SEEM edit. See this tutorial for both SEEM editing instruction and it has the solution to your problem:
http://www.xlr8.us/hofo/
Here is the SEEM to edit:
offset 02 - bit 1: BLOCKS THESE FEATURES: Call barring, Call transfer, Info Services, do not read VM number from SIM, Voicemail ^@ text messages, Multi-Line Support, My Network List, Manual Network Search (1 on, 0 off - set to 0 to UNBLOCK these features) some of these features may be SIM dependant anyway, so activating it on your phone may not actually allow you to use it.