SRTP encryption

OS = CentOS-6.3 (freepbx)
Arch = x86_64
Asterisk = 11.3.0
FreePBX = 2.11.0.0
Twinkle = 1.4.2

I can dial out using a Twinkle soft-phone providing that encryption is set to NO for that device in FreePBX. Otherwise I see this on the soft-phone:

Wed 09:49:41
Line 1: call failed.
488 Not acceptable here

and this in the log:

  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
[2013-05-15 09:55:25] WARNING[1642][C-00000008]: chan_sip.c:10470 process_sdp: Matched device setup to use SRTP, but request was not!
[2013-05-15 09:55:25] WARNING[1642][C-00000008]: chan_sip.c:10470 process_sdp: Matched device setup to use SRTP, but request was not!

Is this a problem with Twinkle, with Asterisk or with my configuration choices respecting encryption on either or both?

On Asterisk the device is set to Encryption = Yes (SRTP Only)

On Twinkle the user profile is set to:

security -> Enable Z/SRTP = yes
  ZRTP -> Only encrypt is remote indicates ZRTP in SDP = no
          Indicate ZRTP support in SDP = yes
          Warn if remote disables encryption = yes

Is encryption in fact available in Asterisk/FreePBX? I am having a similar problem with my test VOIP desk sets; a Snom320 and a Snom870. They have worked in the past with no encryption but when I try to enable SRTP for them I get the same errors as I report above.

I have the snom phones working with encryption but I cannot get the soft-phone to work with it. I infer that this is a Twinkle issue. If anyone has Twinkle working with Asterisk 11 with encryption then I would appreciate being told how it is done or if it is definitely not possible otherwise.