you only have to create a new SIP account for the Grandstream in the T24 and configure a outgoing call profile and put this into the Bell Behaviour.
Admin-Menu: SIP Server Settings
-> Create a new account for this Grandstream
Know you have to put this informations into the Grandstream, so that the Grandstream can register to the T24.
Then go into Admin-Menu: Outgoing Call settings and creat a new profile in which you configure the number which should be called after pressing the Bell Button for example.
- Phone Number: Is the Phone Number of the Grandstream which you configure in the SIP Server settings
- Proxy: Choose the T24 as SIP-Proxy
The Put the OutgoingCall profile to the Remote stations under Bell Behaviour and Video Mailbox.
Be sure that this outgoing call profile is selected in the Signaling phase.
Then it should work like the other Grandstream.