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This is a documented Audiocodes defect. The defect ID is 150893 and is fixed in Mediant version v.7.20A.202.xxx.

The customer to examine the release notes for this version and if there are no changes that will affect them negatively, apply the latest update to their SBC to resolve this issue. 

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Applicable Products: All (SBC). 

https://www.audiocodes.com/media/13232/sbc-gateway-msbr-series-release-notes-ver-72.pdf