Got the latest ATA186 frimware running (3.2(4) with SCCP) and a 2811 with 12.4(24)T5 and a CUCM 7.1.3. So they should be compatible. Other Phones (7945, 7965) register back to the callmanager but these ATA's stay in SRST mode unless they are manually resetted.
I had only one Port in use, so i disabled the other Port with SID1=0 and after that the CM Transistion worked without a problem.
Alternatively you would probably configure the second port on callmanager with a dummy dn as you might oversee or forget this setting on the ATA Webinterface and would run later into the problem to get a device registered on that 2nd port.