From VoIP.ms Wiki
With this feature, you can set a number to be called to by our system, in order to receive a dial tone and place outgoing calls through VoIP.ms. This could be useful if you want to place a call and you are not at home or don't have a voip device to do it in that time.
When a DID is routed to a callback entry and a call is placed to it, the DID number will return a busy signal when called, then the system will callback the specified number and present the user with a dial tone.
You can set this feature at Customer Portal>> DID Numbers>> Callback and you have to fill the following fields:
Description: A brief description for this callback.
Number to call: Here you will set the number you would like to receive the dial tone.
Delay before callback: You can set here the number of seconds the system will wait before calling back.
Response timeout: The maximum amount of time the system will wait to hangup when the user dialed an invalid number.
Digit timeout: The maximum amount of time permitted between digits when user is typing the number.
CallerID Number: This will be the caller ID number for the outgoing call.
After this, you just have to click on the "Create Entry" button to save your settings.
Once this is finished you have to route one of your DID numbers to the callback entry you have created and then dial this number. You will get a busy tone and after that, the dial tone.
Please note this feature also applies for international numbers but is not guarantee that the DTMF tones will work properly, this means that the call may not connect to the destination number dialed after receiving the dialtone.