Yes, very easy if both OBis have a connection to the internet.
Every Obi has a 9 digit Obi number, usually on a label on the underside. On one or both (as desired) OBi set up a hot-line or warm-line to call the other. Insert
<:**9123456789>|
immediately after the opening ( of the PHONE port Digit Map using the actual 9 digit number of the other OBi in place of the 123456789. The **9 prefix means to call over the Obitalk network.
If you want a warm-line that gives you a few seconds to manually dial another number or to replace the handset if taken off the hook accidentally, insert
<S4:**9123456789>|
That gives 4 seconds before automatically calling. Change the 4 for other values of delay.