The recording is being done by the talent, who may or may not be technically savvy enough to produce a high quality recording. After the recording is finished, they must clean it up by removing bad takes, noises, etc. before uploading it to a server, attaching it to e-mail, or mailing it to you on a CD.
ISDN voice-over talent are connected. Ready within hours, sometimes even minutes.
ISDN is NOW. ISDN works. Let it work for you. You just get comfortable.