asterisk users - Jun 2003 - Kphone not working with Asterisk?

I'm trying to get two linux machines with kphone-3.11 two communicate with 
each other over asterisk.  I have them configured correctly on asterisk to use 
sip channels, but when I call from one phone to the other I don't any voice 
communication between the phones.  According to the phones I'm connected,
but
according to asterisk, I get the following message:

 -- Executing Dial("SIP/phone3-2c9f", "SIP/phone4") in new
stack
    -- Called phone4
    -- SIP/phone4-aaf5 is ringing
    -- SIP/phone4-aaf5 answered SIP/phone3-2c9f
    -- Attempting native bridge of SIP/phone3-2c9f and SIP/phone4-aaf5
  == Spawn extension (sip, 2, 1) exited non-zero on 'SIP/phone3-2c9f'

I'm a newbie, so I don't really know whether this is a problem with
asterisk,
or with the softphones.  I've used two Windows estara phones successfully
over
asterisk with the same configuration.  If anyone has any insight on what the 
problem actually is, I would be really appreciative.  Thanks.

Daniel