ISDN may seem like old technology but it just flat works real well for the voice actor and those who employ him. ISDN lines and hardware/software codecs allow for  CD quality voice transmission in real time between studios around the globe. Think of it as a microphone with a really really long cord.

Voice actor and TV news guy Dave Courvoisier has written a good article about ISDN here without getting all technical on you. Nice job, Dave.