If you don't have a call control platform to enable you to do this easily, you would need to configure each of the devices as "standalone" endpoints. You should then be able to make a H.323 call from one to the other using their publicly accessible IP address.
Have a read through the "SX20 as Standalone" thread - it contains the information on how to configure the devices so they can be used this way (the configuration on the SX10 is the same as the SX20, so you can ignore that they are different models for this purpose).
Note: you will also need to configure any firewalls present for each of the networks to allow the traffic on the required ports.
Wayne
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