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 readily available.
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 and then the system will callback the specified number and present the user with a dial tone.
When you receive the CallBack and Dial Tone on a phone please just dial your number and wait for the call to initiate. If you hit your call button on the phone this may result in the call going over your other Provider's Network.
You can set this feature at Customer Portal>> DID Numbers>> Callback Once in there, please click "Create a New Callback Entry" then you have to fill the following fields:
Description: A brief description for this callback entry.
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. If this field is not filled in, it will default to '0000000000' (which may be recognized as Anonymous by some carriers).
Please do not set the same number for your CallerID as the Callback number. Some Call Destinations Require a Valid 10 Digit CallerID in order to terminate properly. Anonymous may not work.
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 it is not guaranteed that the DTMF tones will work properly, this means that the call may not connect to the destination number dialed after receiving the dialtone.