Moving the entry to crontab did not result in the IP being updated. However, after digging around the interwebs for additional information on running jobs in cron.d in Rasbain I came across an entry that referenced that in Raspbian files in cron.d folder should not have an extension.
The fix for running the update utility on a Pi is copy or rename the ip-update-endpointgroup.cron file without the .cron extension. In my case, I did the following to preserve the original file:
root@incrediblepbx:~# cp /etc/cron.d/ip-update-endpointgroup.cron /etc/cron.d/gbip-update-endpointgroup
To validate, I checked the public-facing ip of the router, rebooted the router to force public IP change, checked skytele dashboard to confirm that the endpoint was offline as the public IP had changed, waited for the cron.d to run at next scheduled time and confirmed via skyetel dashoard the ip had changed and the endpoints were on line again.
I will continue to monitor for the next few days, but it seems the script is working as it should for the ip updates.
Thanks for the insights and pointers rampageai.