I've recently moved away from SIP trunks feeding our UC520 over to PRI. Unit originally did not have a PRI card which was installed and additional configs were added to get it working.
One problem we're having is the CFwdAll feature to external numbers.
For example: user sets CFwdAll on his phone to outside number 91xxxxxxxxxx. Caller from outside dials this user's direct number and it rings once and then gets a slow busy. Call never connects. (This used to work when we were on SIP)
Doing a CFwdAll to an internal number works fine. Outside caller is fwded to another inside extension with no issue.