We have an Avaya phone system using IP Office Manager 9.1.6.0 build 153 (IP 500 v2) and Voicemail Pro Client v 9.1.6.0 v2. We have about 4 of 25 Avaya 9508 handsets which do not transmit ring tones when calling external numbers, in spite of the fact that the call is engaged and does get answered by the external call receiver, i.e., the receiver's phone is in fact ringing. Any ideas?