Luckily I met my prof. today. He doesn't use this technical gimmick no more and didn't recommend it.
He had bought the apparatus at a supermarket and had to use a private provider who went broke after a few years. A successor was even more expensive, so he discarded the service and doesn't know whether the successor went broke also.
He was very annoyed by the unsatisfactory abilities of the system, especially the many break downs. He thinks that the modern technique with its inexpensive computers made this system redundant.
Since he needs the internet only for browsing a lot of specialized library catalogues he now uses the facilities of the University Library about ten minutes away.
Browsing through alanabit's link I didn't find such a dinosaur system; our postal services try to be with the vanguard of the technique.

Sorry