Dial cisco 7940G Phone by IP?

I have two Cisco 7940G phones, setup for SIP at the moment but they both have valid LAN IP Addresses, 192.168.1.124 and 192.168.1.125

Both phones are sharing the SIP account and I can use both phones to dial any external phone number..

How can I get the phones to dial eachother?

When I dial the IP address it doesnt work, I tried the MAC Addresses and it doesnt work..

I dont have Call Manager Express, I would like to use these phones as kind of like an Internal Intercom type thing.

Any ideas would be great!

Thanks

Mike