A valid SIP address ( ) or phone number (tel: +41785556677) which will be called.
AlternativeTarget string

As the "Target" field, it should be a valid SIP address or phone number.
If this field defined, it will be used instead of "Target".
This is useful only for the internal purpose when the callback is scheduled based on incoming call
and a caller can enter alternative number for the callback using DTMF signals.
So, in most cases, for the external usage "Target" field is enough.