I have two extensions.
Let's call them extension 1 and extension 2.
Extension 1 has a dial-in number that can be used to call an outside line.

In this environment, I make a call from an outside line to the dial-in number of extension 1.
Extension 1 receives the outside line and transfers the call from extension 1 to extension 2.
At this time, the JTAPI programming tries to get the call forwarding information from the number of extension 1, but it fails to get the call forwarding information from the AES server.
How can I get the call forwarding information with the number of extension 1?

Thankyou for your help in advance.

you say your application tries to query an extension for call forwarding info and the query fails

It would be helpful to provide a trace of the request and the error you get back from the g3trace log on AE Services to provide any meaningful guidance.

See this FAQ for details on logging for TSAPI (JTAPI is a java client side implementation of TSAPI on the server side).
https://www.devconnectprogram.com/site/global/products_resources/avaya_aura_application_enablement_services/support/faq/tsapi/index.gsp

Thank you for your quick response.