If you have multiple devices registered to the Gateway, you can call and transfer calls between these devices or extensions using easy-to-remember short extension numbers.
This feature is available for all Google Voice Gateway accounts.
Start by configuring your SIP devices as described in the SIP Login/prefix article. Specifically, use the GV10000000000x1 format to set up extension numbers (x1, x2, x3) for each of your devices. NOTE: The user name is case sensitive! You must use capital GV at the beginning and lowercase x when specifying extensions.
Your device configured as GV10000000000x1 becomes extension 101, x2 becomes extension 102, and so on.
You can dial between the extensions using the short three-digit extension as well as transfer calls using your device's transfer function.
Three-digit schemes would use SIP IDs ending in x0 through x9 for 100 - 109, or you can set up four-digit extensions in a similar way (x10 becomes extension 1010).