[cisco-voip] prevent IP Phone from up/downgrading FW in LAB when changing CallManagers

[CIPS]

How can I prevent an IP Phone to download a new FW every time I connect it
to a different CCM?

I have a LAB with 4 different IP Phones and one HW platform with CCM 5 and 6
VMWare images.
Every time I change the CCM from 5 to 6 or the other way around the IP
Phones download new FW to do an upgrade or downgrade.
Is there a simple way I can turn this off?

I know of separate TFTP which is shared between CCM's, I also know of
keeping the FW the same on both Servers.
But isn't there an easier way? Service param somewhere maybe?

Thanks in advance.

C.