The phone aerial links to terminals on the (radio) roof aerial. It would have linked to a phone cradle. If you've got any holes or bent bits of trim on the dash, that's probably where the cradle lived. Back in the day, some folks actually cared about getting a good signal for their phone......
The phone kit was an optional extra (as was the CD changer), so the original buyer must have had a few quid spare burning a hole in their pocket.