Online call from a website to mobile phones and SIP
– the Click to Call function

You can place an unlimited number of ‘Call’ buttons on your website. All calls go through Web Call Server and your corporate switch, or through the selected SIP provider. If the user has a headset and microphone, he can make a call directly from the browser by clicking the corresponding link or button on the page.





Calling platforms

  • Chrome
  • Firefox
  • Opera
  • Safari
  • IE
  • Edge
  • iOS SDK
  • Android SDK

Protocols and technologies

  • SIP
  • WebRTC
  • Flash, RTMP, RTMFP

SIP codecs

  • H.264
  • VP8
  • G.711
  • Speex
  • G.729
  • Opus

SIP Functions

  • DTMF
  • Hold
  • Transfer


Online call from a browser using
the Click to Call function

A user clicks the ‘Call’ button in the browser. The browser establishes connetion to Web Call Server and initiates a call to a mobile telephone via SIP-GSM / PSTN gateway of the telecoms provider. Therefore, we receive two-directional voice communication between a user of the browser and a subscriber of the telephony network.



Step by step diagram of the online call from a browser to a GSM phone via the SIP-GSM gateway

  1. The browser invokes the ‘call’ function
  2. Web Call Server converts the call from the browser to the INVITE-request readable by the SIP-GSM gateway.
  3. The SIP-GSM gateway makes a call to the remote callee.
  4. The callee answers the call.
  5. The SIP-GSM gateway sends the OK SIP-status confirming the callee has responded.
  6. Web Call Server returns the ESTABLISHED phone status to the browser, which means the connection is established successfully.
  7. Connection is established between the browser and the SIP device, and the voice traffic is transmitted. The traffic runs through Web Call Server and through the SIP-GSM gateway.



The media traffic between the browser and Web Call Server goes using WebRTC and Flash browser technologies. The traffic between Web Call Server and the SIP part goes via the RTP protocol. Web Call Server takes care of all the required converting and transcoding to provide compatibility of the browser traffic with the traffic received from the SIP device.


Example of using the Click to Call button for an online call from the browser to the mobile phone

Initially, the test button looks as shown on the screenshot. You simply need to click it to make a call to the predefined number.


When the call is established, the voice traffic is sent. To finish the call, click ‘Hangup’.


The mobile phone receives the incoming call and displays the number of the initiating SIP-GSM gateway on the screen.


For more information about testing of online calls from the website and using the Click to Call function, please go to the Testing section.


Download Web Call Server 5

System requirements: Linux x86_64, 1 core CPU, 1 Gb RAM, Java

    Download Now    


  1. wget
  2. Unpack and install using ''
  3. Launch server using command 'service webcallserver start'
  4. Open the web interface https://host:8888 and activate your license


If you are using Amazon EC2, you don't need to download anything.

Launch WCS5 on Amazon EC2


Web Call Server 5 - Trial

The 30-days license is provided once for a person or organization. Please use your corporate email.