SIP->IAX->SIP provider->PSTN IVR DTMF brakes with 1.6(1.8)


Having fight a lot of time with following issue.

We have two asterisks 1.4 and 1.6 and IAX between them
The second asterisk is connected to VoIP provide thru SIP.

When I calling to conference service on some company and dialling DTMF there in some IVR menu al works fine.

But recently we decided to upgrade that 1.4 to or 1.8.4
And we got issues with DTMFs after that.
Now remote IVR is not able to detect DTMF correctly. Sometime detects wrong digits but mostly it just ignore some.
In 5% of times when I am pushing buttons for about 1 sec and keep gap between them about 1 sec I can get success with remote IVR.
But with 1.4 I can push really fast all works perfectly.

With 1.8.4 I even noticed that even our second asterisk on other end of IAX start having issues with detecting digits.

All sip configs have rfc2833 dtmf mode enabled.
I am not changing any configs after upgraded or downgrade. And it always do not work with 1.6 and working with 1.4.31

Other strange thing that in any cases I still can see correct DTMF digits events if enabling debug on outgoing SIP channel to our SIP provider.

Sip provider fr now was not able to address the issues.

Any help will be appreciated.
Thank you in advance