Connecting your office phone system to the public telephone network
First we will briefly explain the term SIP Trunking, what it involves and how you can implement it in practice. SIP trunk is the term for the technology that connects phone systems to the “outside world”, i.e. to the public switched telephone network (PSTN). SIP Trunking technology replaces traditional circuit-switched telephony connections and puts the emphasis on internet protocols.
The SIP protocol (Session Initiation Protocol) is an application-layer network protocol for signalling between two endpoints that exchange voice and/or video. The SIP protocol is used to set up a call or video conference and finally to terminate it when the exchange of data is over. It is most often associated with VoIP technology, although it is not primarily tied to it. Everything runs over your existing internet connection, which is the basic prerequisite for the SIP protocol and its components to work.
The SIP trunk itself provides a logical connection between an IP PBX system (a phone system using internet protocols) and a public telephony operator (PSTN). When a call is made from an extension connected to the phone system to a number in the public telephone network, the IP PBX sends the data to the service provider over the SIP trunk, and the connection to the called number is then set up. All communication takes place over the internet. You can see an example of such a service here.
A practical example:
A company has 2 employees, i.e. 2 phones. Both are connected to the office phone system, and a significant number of calls is expected over a certain period. While connected to the local phone system (PBX), they can call each other locally, but their calls cannot reach anyone outside their local network. Their phone system is simply not connected to a point that leads to the public telephone network. This is where the SIP trunk comes in.
As described above, the SIP trunk logically connects the office phone system, together with all the connected phones, to the other public telephony operators, making it possible to make and receive calls with the outside world. Now you can take advantage of VoIP (Voice over IP) telephony, which is ultimately noticeably cheaper than the “traditional” way of calling. Through a VoIP provider you can call all destinations worldwide much more cheaply and keep track of your spending through customer portals.
Working out the number of channels for a SIP trunk:
With a SIP trunk it is also important to determine how many call channels you need. For many companies this is something of a challenge, as they are not immediately sure how many channels for simultaneous calls they actually need in relation to the number of expected calls, call durations and the number of calls per hour (or another unit of time). This can be estimated in various ways, but let’s take one of the best-known (and most professional) methods as an example – the Erlang method. It uses certain parameters such as the number of calls, the number of phones busy at the same time and so on.
Example:
Put simply, our example would look like this (arbitrary values):
| Number of phones/employees: 2 |
| Average length of a call in seconds: 20 |
| Calls per hour per phone: 10 |
| Allowed failed calls out of 100: 1 |
| Channels needed for the SIP trunk: 2 |
