Check to see whether you have accidentally directed numbers beginning with 5 to an external provider (Zaptel/VOIP) in the Routes administration panel.
To see what is going wrong during the call, log on to the server console using Putty and start debugging:
asterisk -rvvvvv
Then make your call and watch the debugging output. The selintra agi script will start -- see what the notes say. If you can't interpret them, you can copy them from Putty and paste them here for us to read.
EDIT: I think stuntshell and I are saying the same thing. Oops!