POTS is a telecom industry acronym for Plain Old Telephone Service. POTS runs on the PSTN or Public Switched Telephone Network. POTS connections are twisted pair copper wires that run all the way from your phone jack back to the telephone company's central office. These are also called analog lines because they operate on analog voltages and currents according to standards that go all the way back to Alexander Graham Bell himself. Nowadays they may be called landlines to distinguish them from wireless phones that operate on the cellular phone networks.
POTS is solid, reliable telephone service. Voice quality is excellent. It has been enhanced over the years to include options such as call waiting, call forwarding, three way calling, and Caller ID. You can buy a nice business phone for POTS service at any office supply store. If one line isn't enough, you can get a phone that handles two POTS lines.
But what if your business has grown to the point where you need more than two outside lines? Many small to medium size businesses move up from individual phones to phone systems by adding a Key Telephone System. The key is the name for the line selector push button. Every phone set has two to six line keys along with a dial pad and other buttons. It's up to you to decide whether you'll join a conversation on one of the busy lines or select an open line to make a call.
Key Systems are also POTS based. So is the entry level PBX or Private Branch Exchange phone system. The PBX manages outside lines, so you don't have to worry about finding an open one to make a call. It also allows you to directly dial from one phone to another within the business using just a few digits. These calls never use an outside line. They are handled through the PBX system directly.
PBX systems have the ability to grow either by adding on to them or buying larger replacement systems. As your business grows you may barely notice the number of POTS lines creeping up from 4 to 6 to 10 to 20. Users generally have no idea how many lines there are. Management may also not have taken much note of the incrementally growing cost of telephone line service. POTS lines have little economy of scale. It's so much per line depending on the feature set. Saving money is where the idea of the trunk line comes in.
A trunk is a phone line that carries multiple telephone conversations. Modern trunks long ago switched from analog to digital technology. The most popular trunks are T1 telephone lines. T1 is a digital telephone standard that creates 24 individual channels in a strictly timed digital bitstream. Each channel can carry one telephone call. T1 voice lines can do the work of up to 24 POTS lines. The T1 line will plug into a T1 interface card in your PBX system or can be connected to a channel bank that will give you the 24 individual line connections.
Why go to all this trouble? It's no trouble when it results in a cost savings. That's what T1 lines offer. If you are paying for more than half a dozen individual POTS lines, T1 service will likely cost less per month for the same phone service. Once you get to 10 or 12 lines it is almost certain that you can save money by switching to a T1 line.
So, that means there are 24 POTS lines in a trunk, right? Yes...and no. T1 telephone lines can carry 24 phone lines. But many businesses prefer ISDN PRI or Primary Rate Interface service. This is another format of T1 line that gives you up to 23 phone lines. The other channel is reserved for switching and data, so you can have Caller ID capability on your PBX system. There's also something called Integrated T1 that gives you up to 12 phone lines and broadband Internet on one trunk line.
The newest wrinkle is the SIP trunk. SIP is the VoIP signaling protocol. A SIP trunk also carries multiple phone lines and perhaps Internet service. You can trade off voice quality for bandwidth, so a SIP trunk might carry even more than 24 phone conversations simultaneously.
How many POTS lines in a trunk? How many do you need? The trunk can be sized accordingly to give you the lowest cost for the quantity and quality of phone line service you require. Just remember that the purpose of going to trunked telephone service is to save money. How much? Check out the cost savings on telephone trunk lines now.
