You have to perform a traffic analysis on your network to identify the reasons for poor voice quality. Anyway, as you said the problem is not constant, look for bursty traffic on the wan, like ftp, file transfers, backups, database synchronization, etc. If that is the case and you can't eliminate the cause, implement QoS to manage the bandwidth distribution among the services and protocols. Custom queueing combined with packet fragmentation (lower MTU) and RTP header compression often give excellent results.
it is due to high network traffic only. so check for valid data is transfered or some other noise is generated. it can be viewed by the " sh int ser x ".in this report verify the input errors, crc and frames.may be due to virus and network card.
Has this been the case since installation?, if so you may want to move to newer code, i had similiar issues with quality that disappeared with a code update, i went from 12.2(13a)T1 to 12.2(16.1)T, Routers in cloud are 3662's
Introduction: The "external-out enable" command is available for
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