***Cloud Softphone users, please read the note at the bottom.***
For some providers web callback service is already hardcoded in the app. For those providers that are not covered or if you want to override the hardcoded setup you need to set up your own web service.
Go to the Settings > SIP Accounts > Edit Account > Advanced Settings > Web Services
Enable Custom Web Callback section, then enter the URL to your web service. If you are using GET method, just put the parameters in the query string, if you chose to use POST, you can enter your parameters in POST Data field.
You will probably need to use dynamic parameters. For example if you enter %targetNumber% in your URL query string or POST data it will be replaced by the destination phone number. You can also use some account specific parameters. For example %account[username]% will be replaced by the actual username. Or %account[password]% will be replaced by the actual password. One thing to note here is that the password will be passed only if the URL begins with https://, otherwise only couple of stars will be used which won't work obviously. You can also pass sha1 hash of your password istead. It can be achieved by %sha1(%account[password]%)%
Similarly the preconfigured callback number that you can specify in the Advanced Settings section can be referenced by %account[callbackNumber]%
To initiate the web callback make sure you have everything configured properly for the web callback web service, then on the dialpad, or call history or quick dial button just tap and hold - it should show you a menu where you can select Callback action, then it will call your web service and eventually it should call you back.
You can also setup the callback web service for your GSM account. Just go to Settings > SIP Accounts > GSM Account > Web Callback and enter the details in a similar fashion as described above. Of course you cannot reference your username and password here and they need to be hardcoded into URL or POST data.
Callback defined for your GSM Account can be initiated by taping GSM Button. Make sure your GSM Account is in enabled section on the SIP Accounts screen.
Note: Cloud Softphone users should follow the formatting above for the url, but the url should be entered in the cloud softphone web portal under provisioning options, it's not required to enter it in the app for cloud softphone.