SPA series phones are made by Linksys ( now a Cisco company ). These phones can be added to CUCM as 3rd party SIP Endpoints for sure. I've not seen Weltech Phones being added to CUCM as SIP end point but looking at the nature of SIP protocol, any 3rd party phones can be added. Cisco perhaps has not tested it but I dont see any reason why it should not work. Once added to CUCM as 2 different SIP end points, communication would be very much possible among them.
Please try that in your lab.
Hope it helps ?
Please rate useful posts.
GP.