The entire telephone number used to phone a subscriber on a telephone system consists of the basic telephone number and a number in a telephone number block. If the telephone number block is single digit, the direct dial numbers can be 0 - 9. The entire telephone number can then be for example:
The direct dial number 0 of often used as a direct number to the switchboard. Single digit telephone number blocks allow up to ten successive direct dial numbers. Likewise, two-digit blocks can address up to 100 terminals, and three-digit blocks up to 1,000 terminals. The telephone numbers in an assigned telephone number block can be managed and used within the telephone system completely at your own responsibility.
The telephone number block is typically ordered from the telephone service provider. Here it is important to order an adequately sized telephone number block straight away. This ensures consecutive direct dial numbers can be continued in future expansions.
Just as in ISDN system connections, telephone number blocks can also be used in Voice-over-IP telephony (VoIP telephony). VoIP system connections are much more flexible than conventional telephone connections. After all, telephone number blocks can be assigned to an SIP trunk with a simple configuration. The hardware or lines typically do not need to be upgraded for a larger number of telephone numbers. "Fixed Mobile Convergence" (FMC) functions even allow assigning mobile terminals a fixed-line number from the telephone number block. When porting fixed-line numbers to Voice-over-IP providers it is important to note the prefix portion of the telephone number must remain assigned to the company’s registered office and is not arbitrary.