I am sometimes unable to send or receive VOIP communication on highly-populated servers

There is an underlying bug in the UT2004 engine where there can only be 64 simultaneous voice-enabled clients (#887). There is no workaround for this and it is believed to be unfixable without recompiling the engine.

I sometimes crash after alt-tabbing while in fullscreen mode.

The game's engine is very unstable with respect to exiting and re-entering while the game is in full-screen mode.

The development team does not have access to the engine's source code to fix this problem, so the only way to mitigate this is to avoid alt-tabbing as much as possible, or run the game in windowed mode.