Try this solution. Worked for me. (using Xp, msn 6. communicating with other 6 users who use Xp pro and 98)
The problem with version 6....... is most likely caused by a conflict in the versions of rtcdll.dll file that handles most of the IP to IP voice/video streams. Messenger 6 installs a 'bad' version of this file
and they don't 'click'
Also downgrading from 6 to 5...the dll from the version 6 install remains there causing the problem. Same for other downgrades from 6. So that won't solve the problem automatically. Removing the dll that 6 installed before downgrading is necessary. But why downgrade ? 6 is ultimately more fun anyway
What to do ?
For 95/98/ME users...........................
Search your pc for the file RTCDLL.DLL (located in the c:\windows\system32 folder) check the file version by right clicking the file->properties-versions-file version
The good file version for 98 etc is 4.0.3599.0 which is installed by 5 as a default. If you have version 5. something rename the file to rtcdll.old and install MSN Messenger 5. It will install version 4.0.3599.0 to your pc. (or get the file (500kb) elsewhere)
So after replacing the rtcdll.dll in your system32 folder with 4.0.3599.0 version test the voice chat. If it does not work.... ask your counterpart for their operating system and version of the file.
For XP Pro the good file version is 5.1.2600.0 and others may work also. The person at the other end must search their machine for the file as previously described and also replace it.
Also testing the 98 rtcdll.dll in XP (backup your file before replacing) can be an interesting experiment. It should not mess your pc, only your messenger and with any luck and "charm" the damn thing will work.
Tested working voice chat setup (with and without simultaneous video stream) 98 with 4.0.3599.0 version of RTCDLL.DLL and Messenger 6.0.0602 XP Professional with 5.1.2600.0 version of RTCDLL.DLL and Messenger 6.0.0602
Of course before all this you should check if you are behind a router, your firewall setup etc etc, but this solution worked for me and finally made voice chats with and without video in MSN 6 to MSN 6 despite operating system (all my contacts have replaced the files..well almost all) possible.