I have a friend who purchased a few 5121s online. they have version 3.39 on them.
to my knowledge, the last stable version was 3.37.
Is 3.39 the firmware upgrade to connect to the SCME systems?
we're going to downgrade them to 3.37 since they will not connect to his customer's 7400.

I've never seen 3.39, 3.37 is the latest ive been able to find.

AFAIK the ITP-51xx will never work with SCM.

Did he try defaulting the handsets and reconfiguring them?

There is a bug with VLANs and DHCP with ITP handsets

Same thing I said to him. I've never seen 3.39 in any of the phones I've put out. I do recall ordering an ITP via internet and I believe it also had 3.39 and didn't work.
He does have VLAN in the network he is working with, but we downgraded both of them to 3.37 and they worked in my office. I haven't heard from him, so I am assuming it worked for him.
FYI. I did fire up both of them on my network and they wouldn't pull DHCP or register to PBX... I'd really like to know where that 3.39 software comes from, I'd like to "play" with it.