Introduction to SIP Telephony

Today, making or receiving calls through the telephone network (PSTN) is possible via the internet, not just from mobile or landline phones. Special programs called softphones are used for this.

In general, this field for communication between the telephone network and the internet is called IP telephony. It has many interesting tools and capabilities, but in these points, we'll only touch on the topic of calls via the internet, softphones, and accounts for them.

The transmission of calls between the internet and the telephone network is handled by special companies called IP telecom providers. IP telecom providers exist today in almost any country where there is internet.

To get the ability to receive or make calls to the telephone network via the internet, it is necessary to become a customer of one of such communication operators.

Customers of telecom providers can be both individuals and organizations.

To be able to receive incoming calls, you can purchase a virtual telephone number from an IP telecom provider. It is often called a DID number: this is a regular phone number that has a country, city, or provider code at the beginning.

Sometimes an IP telecom provider provides the ability to make only outgoing calls. In this case, the call recipient will see a temporary or even non-existent phone number, which cannot be called back, but this is cheaper than acquiring a DID number.

The procedure for obtaining numbers from different IP telecom providers can vary greatly in terms of sequence of actions and price. In some countries, to get a phone number, you need to go through a complex verification procedure (with a corresponding price).

Usually, telecom providers sell numbers for IP telephony with the code of their country, but often they have partnership agreements with operators from other countries, and they resell numbers in other countries under simplified conditions. This knowledge helps in finding a suitable telecom provider where it's easier to register and where prices for calls to the desired country will be quite reasonable.

In some countries, there are serious legislative restrictions on the use of IP telephony, but there are often ways to bypass these restrictions.

The cost of calls when using IP telephony is determined by the telecom provider, but it is often much lower than that of mobile operators, especially for international directions. Significant cost reduction is possible because the call from the subscriber goes through a shorter chain via the internet, bypassing many intermediaries.

After registering with an IP telecom provider, the user receives a SIP account for connection in a softphone program (they are also called SIP clients). A SIP account usually consists of a login, password, and address of the telecom provider's server.

Sometimes it happens that to connect a SIP account in a softphone, you need to specify a few additional parameters: domain, audio codecs, and communication protocol. These can be found in the personal account or from the provider's support service.

There are many different softphone programs for working on computers, mobile, and other devices, running on a variety of operating systems, from many different developers, including from the communication operators themselves.

Almost any adequate softphone program is suitable for connecting and using your SIP account; there are even a number of good free softphone programs (Zoiper, Telephone, MicroSIP, etc.).

Many telecom providers release their free mobile applications through which you can receive and make calls. In essence, these applications are also softphones, but they don't allow the use of accounts from other providers and often contain many of the disadvantages listed below.

The main problems with free softphones are that often these applications don't work stably (due to the diversity of devices and operating systems, their development and updating is very complex), they require switching attention to them when calling and have an unfamiliar interface, they must be constantly running and drain the battery of mobile devices, they don't support simultaneous connection of several numbers, and may not allow the use of accounts from different communication operators and their simultaneous use.

SIP.TG Softphone is also a softphone program, but it works through Telegram, with SIP accounts from any IP telecom providers in any quantity. The convenience of Telegram for receiving calls is that you don't need to install other applications, the Telegram interface is familiar, its applications work stably on a wide variety of devices, don't waste battery charge, and work worldwide. It becomes possible to receive calls everywhere where Telegram works.

Voice transmission algorithms in Telegram are adapted even for slow and unstable internet connections, which is especially important for use on mobile devices. Classic softphones are much more demanding on the quality of the communication channel.

If the customer of a telecom provider is an organization and it is necessary to give the possibility of using the acquired number to its employees, then a PBX is used (but that's another story).