we are still using UCCE 10.5 and CTI OS for system integration, and was under the impression CTI OS was EOL from 11, but can see CTI OS is still available 12.6, can it still be used in 12.6 or is Finesse the only way for integration?
I don't know about 12.6, but if you look here for 11.6 for instance, it calls out that it is only supported in limited circumstances/configurations.
The vast majority of all UCCE solutions would be using a CUCM PG and the CUCM as the phone system.
In this scenario - CTIOS or CAD could have been used as the Agent Desktop - but now neither are supported as they have been replaced by Finesse.
For some legacy type implementations with ICM and a TDM or System PG - you can continue to use CTIOS - but I would assume the number of actually customers using this are legacy and a very small number.
So its EoL for UCCE <> CUCM .