Hi Sara;
There is a dco which shows that SNOM IP phones can be registerd as SIP end points But not sure about whether they can be used as ccx agent.
It will be easy if you can connect to SNOM support center and very this .
And lastly you need to verify on the uccx licensing part as well i am not sure if some licenses are required for third party phones to be used as ccx agent.
Thanks