So if you click to call, the phone goes off hook, but no digits appear, you have the wrong Application dialing rule built. For example, I had an issue with E.164 sting from the Directory.

+14085552000

This was a directory number in AD. Click to call would be in the Application Dialing Rule:

Starts with +

total digits 13

strip out 8 digits

When actually, the + is not a digit, it does not count this. So it's a 12 digit string. Once I adjusted this, then on CUPS restarted the Sync service.. it worked.

So if you are getting a plank, it's the wrong Application Dialing Rule or issue.