Exchange UM error: Remote participant did not offer required SRTP support

UM was not working for a site and the following error was given by the UM server:

Log Name:      Application

Source:        MSExchange Unified Messaging

Date:          1/14/2015 10:27:09 PM

Event ID:      1079

Task Category: UMCore

Level:         Warning

Keywords:      Classic

User:          N/A

Computer:      server4.mytestlab.com

Description:

The VoIP platform encountered an exception FailureReason = 0

Microsoft.Rtc.Collaboration.CallOperationFailureException:Remote participant did not offer required SRTP support ---> Microsoft.Rtc.Collaboration.OfferAnswerException:Remote participant did not offer required SRTP support

A similar error was given by the Lync client:

A quick Google search did not turn up anything useful. Looking at the Lync client logs, I noticed that the conversations were not encrypted.

Using http://msdn.microsoft.com/en-us/library/dd948772(v=office.12).aspx as reference,

The candidate list in the out Invite did not contain the following three lines:

a=cryptoscale:1 client AES_CM_128_HMAC_SHA1_80 inline:CG6lTObp+llK1yu|2^31|1:1
a=crypto:2 AES_CM_128_HMAC_SHA1_80 inline:X8PSqo+1vhl9R8NxBs1jTVuXNlGr|2^31|1:1
a=crypto:3 AES_CM_128_HMAC_SHA1_80 inline:sHbs569BxAuxz9sVDAVSqQ6|2^31

And the SIP 200 ok showed that the peer connection did not support encryption.

To make a long story short, the issue was a site level Media configuration setting that was configured to "DoNotSupportEncryption." 

Using set-CsMediaConfiguration, I updated the setting to "SupportEncryption"