You might get the above error message when you try to call your voice mail hosted on Exchange 2007 SP1 UM from your Office Communicator Phone Edition (OCPE) powered device. The likely cause of the issue is a mismatch between the VoIPSecurity setting of your SIP URI UM dial plan and the Security - Encryption level setting on the A/V Conferencing properties on your OCS 2007 pool.
The OCPE device use the Security - Encryption level setting to determine, if media should be encrypted or not. The default setting is Require encryption and OCPE will then send media using SRTP. If the UM dial plan VoIPSecurity parameter is set to SIPSecured Exchange 2007 SP1 UM will not accept the SRTP based media and you get the error message above on OCPE. Changing your UM dial plan to have the VoIPSecurity parameter set to Secured will fix the issue. This is the recommended setting, since this ensures that media is sent in a secure way.
More information can be found here http://technet.microsoft.com/en-us/library/bb676409.aspx and here http://technet.microsoft.com/en-us/library/bb124092(EXCHG.80).aspx.