If your extensionsions.conf has this:
[dundi-test-lookup]   
  switch => DUNDI/dundi-test

Then your peer should have the following:
dundi.conf:
dundi-test => dundi-priv-local,0,IAX2,priv:${SECRET}@10.8.?.?/${NUMBER},nounsolicited,nocomunsolicit,nopartial

ext.conf
[dundi-priv-local]
  exten => 5555, ...

Since you don't have ext 5555, "switch => DUNDI/dundi-test" causes asterisk to
check the line in your peer's dundi.conf
     dundi-test => dundi-priv-local,0,I...

It will then lookup [dundi-priv-local] un his ext.conf, find 5555
and ring to it.