Binding Voiceall to a key

While in game, open the console (~) if that doesn't work you have to go to Options > Keyboard > Advanced > Enable Developer Console, then press ~.

Type:

bind voiceall

i.e. bind h voiceall

Remember that voiceall is a toggle, when you enable it, holding down the V key (or whatever key you remapped it to) now talks to the whole server instead of just your team. Use it, and turn it off when you are done using it.