Settings and use of the DTMF keyboard when Telegram- calls

To open the DTMF keyboard during a call, swipe the screen up or left, or press the Back button. The keyboard is sent to the chat with the user @siptg (in Softphone mode) or to the chat with the gateway account where the call history is contained (in Gateway mode).

When setting up SIP accounts from some telecom operators or registering with a PBX whose settings we do not have in the list of Server Templates, you may need to manually specify one of three methods for the DTMF keyboard: RFC2833, inband и INFO (all methods except inband send confirmations).

RFC2833 - the main method of transmitting signals about pressed buttons in SIP networks. However, due to freedom in implementation, some systems turn out to be incompatible with each other (in this case there are INFO and inband methods).

inband - this is when a button press tone is transmitted in a common stream with voice, but due to the audio codecs used, it can be distorted and the receiving side does not recognize it. Therefore, this method is usually used only as a last resort.

INFO is one of the newest ways to transmit a DTMF signal. On the one hand, the method is quite reliable and easier to implement than RFC2833, but due to its novelty it is not supported by all operators.

The required value is indicated without spaces, the case of letters is not important.