A vague question, no doubt, with little info on my part but . . .
In-house VoIP setup with Astrisk, Grandstream 2000 phones, Digium TDM2412E and S101I - several places my customer calls can receive and transfer calls from them fine if they call out on “regular” phone line - i.e. fax phone, etc. However, if they call out on their individual Grandstream to the same contacts, same phone number, when the call is transfered to their voicemail the call is disconnected.
Ever heard of it? Any ideas? What info do you need beyond these tidbits?