Calling from Bochum to Hamburg in late 1980s?

The other day, I was reading the memoirs of a German theater producer. In it he talks about how in the late 1980s, he established Starlight Express in Bochum. Initially, he says, ticket sales were slow because the Teleticket hotline to purchase them was a number for Hamburg and not Bochum. But once a call center was established in Bochum, they picked up significantly.

Does anyone know why calling Hamburg from Bochum would have been such a hassle? I've been digging around but can't find anything.