I just had an installation that was using a selector code to Centrex transfer to another office. It was working great, then all of a sudden, calls would not complete. Monitoring the line with a butt-set revealed that the Partner Messaging would disconnect BEFORE the call to the outside number completed. Adding a PAUSE (* when programming the outside number) made it work right again. Maybe they changed LD carriers? Don't know.
So, monitor the line with a butt-set and see what is happening. If you need to add pauses at the end of the number, don't add too many; the person at the far end might answer and only hear silence until the PM releases.