Do you have any information on the Motorola VT1000 with the Cisco call manager? I am currently using the Cisco ATA, but the VT1000 caught my eye.
The Cisco ATA supports multiple protocols, including SIP, H.323, MGCP and the Cisco proprietary Skinny Client Control Protocol (SCCP). It is likely that you are currently running the Cisco ATA with the Cisco Call Manager using SCCP. The Motorola VT1000 only supports SIP. Cisco only recently announced SIP support for the CallManager:
One piece of the article says: "Cisco says SIP support on CallManager does not include SIP endpoints, such as third-party...
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SIP phones, or softphone clients, but this support will be added in the future."
So you probably need to contact Cisco to find out when they plan to release support for third-party SIP devices such as the Motorola VT1000.