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This can be caused when a phone does not send the entire dialed string 
"*97" to the PBX. The root cause will vary by phone model. The solution 
is to modify the dial string for the phone such that it sends unaltered 
*xx codes to the PBX. If you need assistance configuring a dial string 
please provide the phone model.

 

Also, the reason it may have appeared to work before is that some phones
will properly send the *97 if dialed before engaging dialtone. For 
example: with a Polycom phone, dial *97 and then pick up the handset. 
This method will bypass the phone's internal dial string logic.