When PSTN user calls Cisco IP phone user, no ring tone is heard on Cisco IP Phone
You haven't specified whether you are using CUCM, CME, H323, MGCP or anything else that might help diagnose your issue...
However, it's most common to get this issue on H.323 based systems. Take a look at this document: http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a0080094c33.shtml#ringback_pstn
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