There are certain situation issues which can arise when interoperating signaling capabilities between the gateway and Telco, one common problem is no Ringback tone.
Configuring progress_ind should only be configured when necessary. Please review the following link, it describes in detail issues such as the no ringback tone on PSTN calls, and how to troubleshoot and resolve it using the progress_ind command.
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a0080094c33.shtml
Regards
Allan.
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