FAQ/Why do I hear a beep before ringing sound when making call
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After dialing a number and pressing the dial key, you might hear a short beep before the ringing sound.
This problem occurs because some IP PBXes (such as asterisk) send "180 Ringing" followed by "183 Session Progress" and in-band audio. After receiving "180 Ringing", the phone starts generating the internal ringing sound for a short while, until receiving "183 Session Progress" (you perceive it like a beep because it is usually generated for less than a second). After receiving "183 Session Progress", the server starts sending the actual in-band ringing sound, which you also hear.
In order to avoid this problem, a correct behavior of the PBX would be to either send only the "180 Ringing" SIP message and no in-band audio, or "183 Session Progress" + in-band audio.
You should contact your PBX administrator or search the documentation of your PBX in order to correct this behavior.
Solution for asterisk servers
On asterisk servers, you can change this behavior by setting "progressinband=no" or "progressinband=never" in your sip.conf configuration file. For more information on this parameter, you can check the following documentation: http://www.voip-info.org/wiki/index.php?page=Asterisk+sip+progressinband