What's My Line?

If you are like us and upgrading from a very old PBX you might want to change your phone circuits. Here are the options:

POTS or Plain Old Telephone Service: Voce quality lines. You can buy one at a time. I don't even have this at home. We will have 3 POTS lines for our fax machines and modems. Each POTS line has a phone number assigned to it. Fun Fact: In many countries POTS used to stand for Post Office Telephone System because their post offices also ran the phone system.

Analog T1: Similar to POTS in that it is voice quality line. We have 2 T1 right now, one supplies 23 lines to our PBX, the other one supplies 8 DID lines, some analog lines that don't go into the PBX (faxes, modems). Each line has a phone number assigned to it. With the analog T1s were were paying a base monthly fee plus usage charges. Each line goes to our phone patch panel and gets connected to a separate port in our PBX.

T1 PRI: Stands for Primary Rate Interface. A digital phone line carried over a T1. Tried and true technology. I strongly considered moving to this technology, it would have saved us over $200/month over our analog T1s plus it would have bundled most of our usage. Each PRI would be a single cable that would plug into a PBX. Much simpler than the analog T1. Our current phone system does not support a T1 PRI but does support ISDN PRI.

SIP Trunks: We are going with this technology. Considered new in terms of phone technology but gaining ground. This is all digital. SIP Trunks can use a T1 or a metro connection. The number of simultaneous conversations it can support depends on the codec you choose, we are using an industry standard one and are paying for 30 channels on a T1. You can squeeze more traffic on a T1 but at the cost of lower quality by choosing a different codec.

One advantage is the ability to run voice and data traffic over the same pipe. Many vendors will offer to dedicate a certain bandwidth to voice and the rest to data. Some vendors warned me not to mix voice and data, others don't see a problem.

One disadvantage is that you shouldn't try to run fax over SIP trunks, some vendors claim to support it but I was warned that the results are often poor.

This is less expensive than the T1 PRIs. We will have all or most of our monthly usage bundled into the base price.

Being true digital there is no no relationship between the number of phone numbers and the number of channels. We could have just 5 numbers and 30 channels. Once everything is working we should look into eliminating some of our phone numbers.

Be sure that the SIP trunks you get are compatible with your PBX. Like all standards there is wiggle room and some SIP implementations have proven to be incompatible. It is cases like this one where phone guys laugh at computer guys.

DID lines: Direct Inward Dialing. This is really a service. This allows you to give different inbound numbers to different phone users without getting additional lines or regular phone numbers. DIDs are usually sold in 20 packs are are inexpensive. In our current system we have 8 DID lines, so our PBX can handle up to 8 people calling in on direct lines simultaneously. The 9th caller would be routed to our operator. With our new phone circuit and PBX we will be able to handle as many DID calls as our basic line can handle. In addition, we will have the option for outgoing calls to show the DID number instead of the number of the phone line it goes out on.