Transfer calls with Cisco 7940 result in Zombie channels

Since I have upgraded to Asterisk 1.4 transfering incoming calls from my Cisco 7940 phones result in Zombie channels.

I have a few Cisco 7940 phones a Snom320 and some ATA’s and softphones.
The strange thing is, that when transfering incoming calls from any phone to an other except the Cisco’s it will works all ok.

When I transfer an incoming call from a Cisco to an other it fails.
This is what happens in the CLI:

 -- SIP/500-08495bf8 answered IAX2/sp01-2
    -- Started music on hold, class 'default', on IAX2/sp01-2
    -- Executing [520@office-grp:1] Dial("SIP/500-0848d290",
"SIP/520|20|Ttr") in new stack
    -- Called 520
    -- SIP/520-0849fd28 is ringing
    -- SIP/520-0849fd28 answered SIP/500-0848d290
    -- Started music on hold, class 'default', on SIP/520-0849fd28
    -- Stopped music on hold on IAX2/sp01-2
    -- Stopped music on hold on SIP/520-0849fd28
  == Spawn extension (ivr-office, s, 2) exited non-zero on 'SIP
  == Spawn extension (office-grp, 520, 1) exited non-zero on
    -- Hungup 'IAX2/sp01-2'

When this happens the calling party doesn’t hear anything, but the receiving party does.

When I do exactly the same but skip my Cisco phones there is no problem.
Does someone have any clue what is going on?

I have upgraded to the latest SVN of Asterisk, libpri and zaptel.