[Asterisk-Users] 2 sip servers

[Altus Snyman]

Good day all
I have figured out most/basics of asterisk.I went with sip and made my
own sip.conf and extensions.conf

No I have 2 servers running sip in different towns.Both is connected
with static ip so thats fine,but now.
Lets say someone want to call someone else in the other town.How do I
get asterisk to know,for instance sip extension 101 is on another sip
server on a different ip.
And I even want to say if someone in town A calls a town B code it
should go out threw town B's pstn card,so it will only be charge for a
local call?Can this be done

Its hard to explain for me and my english is not that good.Please Help
Thanks
Altus