When an outbound call is placed through the Session Initiation Protocol (SIP) PRI Gateway to public switched telephone network (PSTN)/Cellular Caller, some calls are dropped if the called party does not answer, when the call is switched to voicemail. When calls are placed repeatedly, it sometimes goes successfully to the voicemail. When the call drops, caller hears a fast, busy tone.
It is observed through the debugs that if the gateway does not receive a connect from telco within 30 seconds, the SIP server or proxy disconnects the call. This call, if successful, shows the receipt of a connection from telco before the SIP server times out in 30 seconds.
In order to resolve this issue, increase the SIP Timeout to accomodate all the call.