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What kind of T1 line should I order from my phone company?

Answers to common Plum iOn questions

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What kind of T1 line should I order from my phone company?

Post by support »

You generally have a choice of T1 line protocols from your telephony provider. There are two major types of T1 lines. The cost and features of each type of service vary by telephony provider.

channelized T1

Also known as cT1 or "robbed bit" T1 line, because the system "steals" bits from the data stream to handle signaling. Each cT1 line provides 24 ports.

Channelized T1 lines are offered with a variety of framing and encoding options. The Plum Voice platform supports SF (D4) framing and AMI coding. It also supports ESF framing with B8ZS coding. This is sometimes referred to as "level 1" signaling.

"Level 2" signaling provides call control signaling on top of the basic line synchronization. The Plum Voice Platform supports:

- loop start
- E&M immediate, wink, or double wink start.
- ground start (inbound only. outbound and call transfer not supported.)

Note that some telephony providers offer ANI and DNIS service only with E&M or ISDN service. Not all providers offer service with all protocols.


ISDN provides 23 ports per T1 line. The 24th channel is used for out of band signaling. ISDN lines typically support both ANI (caller ID)and DNIS (dialed number information) service.

The Plum Voice Platform supports the NI2 variant of the ISDN protocol.