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Hissing sound and no voice after IOS upgrade!

Hi, we have lots of branch routers which are 2811 with FXS and pvdm2-8 modules . our voip network was fine and working properly but after we updated our router ios from 12.4(24)t to 15.01-M10 we have strange problems over our voip network. after upgrade we have some faulty routers that when we try to make call we have Hissing sound(only hissing and no other sound) or even no sound at all so we can not make voice calls at all. I dont Know WHY this is happening because i have two identical routers with same config same modules and one of them is OK even after upgrading to ios 15 but other one with same config and does not work! can you guide me where is the problem? do we have major change in voice structure in IOS15 or im missing something. thanks in Advance

Note: we are using g729 codec and we have Routers that are OK and their VoIP is working even after ios upgrade.

also i have gone through all caveats related to IOS 15.01M10 but i didnt find any thing relevant.

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