You need to understand the 2 things
- Contact Center (Cisco UCCE, UCCX, PCCE)
- Underline IP Telephony (CUCM)
JTAPI works with CUCM (IP Telephony) and this doesn't have a notion of the contact center. Finesse is the part of Contact Center , so callVaraibels are only available in the contact center domain not in the IP Telephony domain. So won't have the CallVariable in the JTAPI.
Hope this helps.