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One voice jack can support four analog/Centrex phone lines. One Data jack can support up to 2 VoIP phone phones.
VoIP Phones
For VoIP phones, the device can essentially have an unlimited number of phone lines associated with. By installing a 5 line phone and then adding on sidecars you can ensure that multiple lines will appear on the single phone set. Up to 2 of these sets can be placed on a single data jack
Analog/Centrex Phones
For analog/Centrex phones, in one jack you can install four phones on the voice port of the jack. For example, two phones, one fax, and a credit card machine.