asterisk users - Apr 2007 - Phones working with 1.2.17, not with 1.4.2

Hello,

I've got various phones (mostly SPA-922) behind NAT registered to
Asterisk. I've set nat=yes and canreinvite=no, and everything seemed to
work great with 1.2.17. After upgrading to 1.4.2 using users.conf and
macro-stdexten my spa-922 can't call other extensions.

    -- Executing [23@default:1] Macro("SIP/22-b72006f0",
"stdexten|23|
SIP/23") in new stack
    -- Executing [s@macro-stdexten:1] Dial("SIP/22-b72006f0",
"SIP/23")
in new stack
    -- Called 23
[Apr 18 12:29:16] NOTICE[3831]: chan_sip.c:2757 auto_congest:
Auto-congesting SIP/23-081db528
    -- SIP/23-081db528 is circuit-busy
  == Everyone is busy/congested at this time (1:0/1/0)
  == Auto fallthrough, channel 'SIP/22-b72006f0' status is
'CONGESTION'

Debugging SIP messages seems that the called exten is not replying to
invites, but it registers correctly. Other phones (Siemens C450 IP) seem
to be able to call other extensions:

    -- Executing [22@default:1] Macro("SIP/27-b72020e0",
"stdexten|22|
SIP/22") in new stack
    -- Executing [s@macro-stdexten:1] Dial("SIP/27-b72020e0",
"SIP/22")
in new stack
    -- Called 22
    -- SIP/22-081de4f8 is ringing

Phones configuration is unaltered. What could it be?

thanks

Luca