There is a keyboard shortcut to each of the 5 different tunable radio's.
Radio 1 is / has Bruv mentioned
Radio 2 is SHIFT+ /
Radio 3 is CTRL + /
Radio 4 is ALT+ /
Radio 5 is for voice communications, but you can also use text. SHIFT + \
There's a quick reference chart on the Trainers website with the keyboard shortcuts, it's a little old but I don't think much has changed in regards to the default key mappings. http://trainers.hitechcreations.com/quickref/quickref.pdf