DUNDI is set-up between sanr-pbx and shef-pbx with the same config. sanr-pbx (asterisk 1.8) can dial shef-pbx (asterisk 1.4) extensions fine but not the other way round.

The sending server (shef-pbx) sees this:

    -- Called dundi:tdoGONUtJVzZTMY2WgVFBg==@sanr-pbx.xxx.com/240
Tx-Frame Retry[000] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW    
   Timestamp: 00014ms  SCall: 05615  DCall: 00000 [173.164.174.50:4569]
   VERSION         : 2
   CALLED NUMBER   : 240
   CODEC_PREFS     : (ulaw|gsm|alaw)
   CALLING NUMBER  : 708
   CALLING PRESNTN : 0
   CALLING TYPEOFN : 0
   CALLING TRANSIT : 0
   CALLING NAME    : Someone
   LANGUAGE        : en
   USERNAME        : dundi
   FORMAT          : 4
   CAPABILITY      : 3989518
   ADSICPE         : 2
   DATE TIME       : 2012-03-14  17:54:04
pbx01*CLI> 
Rx-Frame Retry[Yes] -- OSeqno: 000 ISeqno: 001 Type: IAX     Subclass: REJECT 
   Timestamp: 00014ms  SCall: 00001  DCall: 05615 [sanr-pbx.xxx.com:4569]
Tx-Frame Retry[-01] -- OSeqno: 001 ISeqno: 001 Type: IAX     Subclass: ACK    
   Timestamp: 00014ms  SCall: 05615  DCall: 00001 [sanr-pbx.xxx.com:4569]
    -- Hungup 'IAX2/sanr-pbx.xxx.com:4569-5615'

The receiving server (sanr-pbx) sees this

IAX2 Debugging Enabled
Rx-Frame Retry[ No] -- OSeqno: 000 ISeqno: 000 Type: IAX     Subclass: NEW    
   Timestamp: 00015ms  SCall: 11574  DCall: 00000 [shef-pbx.xxx.com:4569]
   VERSION         : 2
   CALLED NUMBER   : 240
   CODEC_PREFS     : (ulaw|gsm|alaw)
   CALLING NUMBER  : 708
   CALLING PRESNTN : 0
   CALLING TYPEOFN : 0
   CALLING TRANSIT : 0
   CALLING NAME    : Someone
   LANGUAGE        : en
   USERNAME        : dundi
   FORMAT          : 4
   CAPABILITY      : 3989518
   ADSICPE         : 2
   DATE TIME       : 2012-03-14  17:52:18

Rx-Frame Retry[ No] -- OSeqno: 001 ISeqno: 001 Type: IAX     Subclass: ACK    
   Timestamp: 00015ms  SCall: 11574  DCall: 00001 [shef-pbx.xxx.com:4569]