[Asterisk-Users] call another server

[Altus Syman]

Good day all
We have a few of asterisk server running sip and zaptel/voicetronix cards.
All the server works 100 for calling in and out of the pstn and internally
Some of the server are owned by the same company so I configured IAX so 
they can call different extensions at different branches
Now,how do I configure asterisk so that if someone calls there public ip 
asterisk will answer as if its coming in from the pstn
I.o.w I want to phone a IP instead of a phone number
Can this be done?
Thanks
Altus