Using ATA on 3CX is easy once you upgrade to the latest firmware, SIP version 3.01.
Get that firmware here:
http://kvin.lv/pub/Cisco/ata_03_01_00_sip_040211_1.zip
Upgrade instructions are in the zip package.
Here is the forum link where I found the info:
http://voip-forum.tmcnet.com/forum/forum_posts.asp?TID=789
Good Luck
Spike