Symptom:
Currently, the UCCX 10.5 Release Notes and SRND state:

"Finesse supports the use of multiple lines on agent phones. You can configure one or more secondary lines on an agent phone. The first four lines (one ACD line and the next three secondary lines) are monitored for setting the agent's state."

This is confusing  as it seems to suggest that the ACD line must be in the first button position.

Conditions:
The ACD extension of the UCCX agent must be in button positions 1 - 4 and other, secondary, non-ACD lines can be in the other positions. The first four lines on the phone are monitored for setting the agent state, regardless of whether these lines are ACD or non-ACD.