Cisco doesn't test this so you will need to test/verify this works on your own. They publish what they explicitly tested under the UC Compatibility Tool. Remember that SCCP has an internal versioning mechanism as they make improvements. Both CUCM and IOS/SRST need to understand the SCCP version the phones are running. What you're expecting here is a fairly significant version gap between IOS and the current firmware to not have problems. It may work but if it doesn't, Cisco is going to tell you to upgrade IOS.
Also be aware that SRST 4.0(1) was on IOS 12.4(4)XC. This was part of the 12.4 mainline release which has reached end of life.
My advise is to upgrade the routers to a newer IOS release; I believe that the ISR G1 hardware can at least make it to 15.1(4)T which would be SRST/CME 8.6. Be careful to deal with memory/flash requirements which were increased around the 12.4(20)T timeframe.
