The HR10-250 has two distinct phone calls.
1 call to local POPs that connect the unit basically via the "Internet" to upload/downlad the TiVo software specific data
1 call to DirecTV 1-800 to upload PPV information.
The TiVo portion of the call is what you see when you "force" a call.
And the portion that gives you the "failure" messages.
There is nothing on the system to tell you if the 1-800 DirecTV call fails or succeeds...
Anyway...
Part of the problem is sending DATA over the vonage. Vonage compresses the data. And if the connection simply isn't clean, or the modems on the POP side of things are not up todate, ect.... there are multiple points that it can fail.
The HR10-250 modems do work with vonage, in a lot of cases... just not all.
As for the HR20...
Very different machine. But from the "phone" call side of things... It just does the 1-800 DirecTV and seems to have NO problems with any of the major VoIP services.
I have had mine on Vonage now since I activated the box... and not one PPV hasnt been billed..