A Callback system allows the user to 'receive' a call instead of making an 'outgoing' call. This is useful in many situations where making an outgoing call:
Is not allowed: e.g. if international or outgoing calling is barred.
Is very expensive: e.g. through cell phone packages with high outgoing minutes and airtime charges while incoming minutes are free.
Is not desirable: e.g. in a situation where it is not desired to pass the CLI
Is blocked: e.g is a country where VoIP is banned or regulated
In such situations, the subscriber sends a "callback request" to the call back SIP application running on top of Application Server. The SIP application makes a call back to the subscriber which is an incoming call to the subscriber. This is called Leg A of the call. The SIP application then dials the called party. This is called Leg B of the call. The two legs are then joined together so that the two parties can talk.
Methods to accept CallBack requests:
CLI reading DIDs
This involves publishing a DID number that has the capacity to read the CLI numbers for all incoming calls. When a subscriber wants to receive a CallBack requests, he rings that DID number. The DID number reads the CLI and drops the call without picking up so no call is charged to the subscriber. The CallBack SIP application can then initiate the CallBack to the subscriber and ask for the CalledNumber which the subscriber can enter through his keyboard i.e. over DTMF signalling.
This method requires that the subscriber's phone system should be able to pass the CLI on its call to the published DID and once the incoming call is received, the system should be able to pass DTMF signaling so that the dialed digits can be passed.
Dedicated DID CallBack
In case there is a restriction or problem of passing the CLI to the published CallBack DID, then the CallBack DID is not able to read the CLI when a subscriber rings that CallBack number to request a CallBack. In such situations, it is possible to issue one dedicated phone number i.e. DID to each subscriber. This typically costs $0.50 per month per phone number but allows the operator to uniquely identify the subscriber because of his unique DID and can initiate a CallBack. This system does not require the capability of passing CLI to the CallBack DID. This system still requires that the subscriber is able to pass the DTMF to dial the Called Number.
In this method, the subscriber comes online and requests the CallBack through a web based interface. In this case, the subscriber can enter his own number as well as the number that he wants to dial and the CallBack SIP application can initiate the CallBack to the originator subscriber and then dial the Called Number directly without the need to read any DTMF. This is a very efficient method for doing CallBacks.
This method is same as the Web CallBack except that in this system, SMS is used instead of online web based request. In a SMS message, the user writes his desired called number and sends the SMS. The CallBack SIP application, then initiates the CallBack to both legs and join them. If using other people's phones, the user can optionally send the leg A phone number as well as his dialing PIN for authentication of the request.
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