Hi. Have you resolved this issue?
Do you have a FW between ATA and CallManager? For the ATA you have to open SIP comunication.
When you connect the ATA to a Cisco switch, you can use CDP to verify ATA IP Address. Then use a browser to navigate to the IP Address of the ATA.
Verify with RTMT the TFTP logs of the CallManager, to see if the ATA gets there.
Hope this helps.