I noticed something that I thought I would share for the benefit of others.
FreeSWITCH Version 1.5.15b+git~20141215T224714Z~0b414a8de8~64bit (git 0b414a8 2014-12-15 22:47:14Z 64bit)
place a call with linphone using opus to a freeswitch server voicemail extension.
listen to the outgoing message and leave a voicemail after the prompt.
Fairly loud comfort noise is heard while leaving the message.
should hear silence instead of comfort noise.
If we change the codec to pcmu we hear silence instead of comfort noise.
Comfort noise is heard because after the outgoing message freeswitch stops sending rtp.
add this to the voicemail extension before the answer application.