On the HD2 page there is a note that reads
"NOTE: Each Ooma Telo can support up to 4 wireless peripherals. The HD2 Handset is not compatible with the original Ooma Telo Handset or the Ooma Hub. Some features require a subscription to Ooma Premier."
Now I don't know how you would interpret not compatible, but my reading of that was that the original HD1 handset would not do what the HD2 can do so they were not compatible.
The reality is, if you buy and upgrade the Telo to work with a HD2 handset, your old HD1 handset(s) will not longer function with the Telo! Period! So much for any investment you might have had in your HD1 handset(s).
So if you think you can just add a HD2 to your existing HD1 handset or sets, that is not possible.
You have been warned!
1/ (with) incapable of existing or living together
2/ unable to be used in combination
3/ not consistent.
a. (of software) NOT capable of being run on another computer without change.
b. (of hardware) NOT capable of being connected to another device without the use of special equipment or software.
5. Electronics.(of a device, signal, etc.)NOT capable of being used with equipment in a system without the need for special modification or conversion.