Environment: The Avaya BDE is 6.0, and the dmcc api is 5.2.3
I understand that the mobile phone number to ring regards EC500 is configured via "change off-pbx-telephone station"
Through DMCC API calls (directly or via phone display) is there a way of getting the forwarded mobile number configured for EC500?
If not, non DMCC API method exists to get it?

Take a look at AE Services System Management Service (Web Service). Through it you can access provisioned information on Communication Manager. In looking at the current release guide you can access that form (SMS calls them 'models'). One of the models is 'OffPbxStationMapping' which is what I think you are looking for.

http://www.devconnectprogram.com/fileMedia/download/f5bae263-b945-4064-9feb-17d4ae7b6e41


http://www.devconnectprogram.com/site/global/products_resources/avaya_aura_application_enablement_services/releases/6_3_3/index.gsp
Click Downloads, then Web Services and then look at the bottom of the page for the downloads.

you said you were using BDE.. which is 6.1 AE Services. so this is the right SMS programmers guide.
http://www.devconnectprogram.com/fileMedia/download/2bde102d-1143-4cc5-b25d-64ed6ae4b994

and yes 'OffPbxStationMapping' was supported by the SDK in that release so you should be able to prove this out.

Thanks for the reply.
Shame I can't do this from DMCC and have to use SMS