ISDN is an abbreviation for Integrated Services Digital Network and is a type of telephone line.

ISDN offers advantages over the standard type of telephone line, as it is possible to make two phone calls at once and to have up to 10 numbers, although normally only 3 are assigned.

In the days before internet telephones and cheap mobile phones, this meant that a family could have a number for each person and even if someone made a long phone call the other line would remain available. Small businesses often use ISDN lines to separate telephone and fax calls.

ISDN also offers additional services such as conference calls and call re-direction (“follow me”).

Normal telephones require an adapter to work on an ISDN line, known as an “A/B-Wandler”. The line itself is a digital connection to the telephone exchange and the numbers dialled are transferred digitally as opposed to using DTMF tones or pulses.

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Das Festnetz is the name given to the cable-based telephone network, known in English as land lines or PSTN.

Telephone numbers in this network have a set Vorwahl (Prefix or STD-Code). Previously the numbers were often assigned to a particular property, these days it is usually possible to take your number with you as long as the area code stays the same.

Apart from the Deutsche Telekom some cities have their own cable-based telephone networks, but there are also two larger completely separate networks owned by the Deutsche Bahn and the German army.

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A Flatrate is a method of paying for telephone calls or for internet usage.

Generally a flatrate will mean that all calls to a particular network or destination are included in a fixed monthly price. Similarly, all internet usage may be included in that price.

Some flatrates do, however, have restrictions on them such as a maximum amount of minutes or Gigabytes that can be used in any one month.

Generally a combined telephone and internet flatrate will cost between 30EUR and 50EUR, although if you are only staying in Germany short-term then you should check the conditions carefully as most providers will want you to take out a 2-year contract.

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