Welcome to the world of Freepbx.
A few things which I would suggest, One to change the ip of your PBX to a static address, as if the ip is reasigned then your sip phones will not know where to connect to.
This is simple to do, Log in to the box via terminal or putty
at the command prompt type setup
go to the network configuration and put in a ip address that is outside the scope of your dhcp server but on the same subnet, also ensure you put in the gateway and netmask and dont forget dns server as you will need this to resolve for system updates.
after you have done this exit out of the config editor, and type /etc/init.d/network restart
you will now have to log into the box if on a putty session on its new ip.
to get your sip phones to work use the following config the ip address of the asterisk server, your ip phone will have to get a ip and gateway address but you can allow your dhcp server to do this.
ensure that you include the IP address of asterisk as the registrant address port 5060 is normally used for sip and dont forget to include a password.
Ensure your sip config had the phone no + password configured, dont worry about email or any other settings for the moment.
if you are unsure that you phones are registered, you can normally see them from your web interface, But to be certain, log into the terminal on the console or via putty
type asterisk -rvvvvvv
type sip show peers
this will show you any sip clients that are registered.
Hope this helps
Oh BTW make sure when you do config changes that you apply the changes with the apply changes bar.