There two things here, dialing by IP address and hotline dialing. The trick is putting the two together.
You can do direct IP dialing using the keypad on the phone. There's another FAQ posted on how to do IP address dialing.
In this case we need to change tell the SL4020 not to look for a SIP server and to set up a hotline call. Here is how you set up hotline dialing by IP address -

1. Under Telephony, SIP - Set the SIP Registration Server Address: to the IP address (or domain name) of the Smartnode / FXO gateway. Save the changes on this screen. This setting directs the Smartlink to send the call to the FXO gateway.

2. Under Telephony, SIP - Uncheck Send Registration Request with Expire Time:

3. Under Telephony, phone one (or two), user information.

   1. Set the phone number to the number to be used for phone one. (For example: 555 for line one).

   2. Set the Caller id to be used for phone one.

4. Under Telephony, phone one (or two), supplementary services set "IP dial srv " to YES.

5. Under Dial out type, set to Hotline and code the SIP user part of the SIP call header. This will be combined with the IP address set in step #1 to form the complete SIP header.

6. Save all the changes.

7. Repeat steps 3-5 for phone 1 using a different phone number (For example: 444 for line two).

8. Reload the SL4020.

When the handset is lifted, a SIP call will be made to the destination sip address.
SIP calls placed to 555@IP address of the SL4020 will ring phone one.