IAX and PSTN

Hello,

I have various asterisk servers comunicated with IAX, and my question is, that if I have one server, for example servera and other, serverb, how can I do a call from servera to the pstn of serverb (that the call to the pstn goes from serverb).

Thank you.

Its not for commerce, its for our company, we have delegations in diferent places.

Thank you again.