The following text is from a post that I saved, I forgot who the author was nor am I sure it applies to you, but it may help (the part referring to the addition of commas and *99 may help it from having so many errors and retries if it works as advertised).
The following is quoted from someone else, none of the following is from me.
I recently got in on the $200 HD-Tivo deal, as I'm sure a lot of other people have done as well. I love it but I noticed that it HATES calling home with Vonage, and I couldn't get mine to do it. There was always some sort of error message. After reading a bunch of threads in another forum, I finally found a way that works for me. Here is what I did:
I set the prefix to: *99,12122773895,,,,, as per another poster's suggestions in the Tivocommunity forums. Note there are 5 commas (pauses) at the end. I left the phone number as 1-212-277-3895. I didn't put anything into the call waiting prefix.
Supposedly the *99 tells Vonage that the call is a data call and the comma's (pauses) at the end are to slow the modem speed in the HR10-250 way down. This setup worked for me last night, although it took a few tries for it to connect. I was able to get the newest software downloaded which was my main goal anyway.
Don't be dissapointed if you see the status of the daily call as "Downloading" and it seems stuck there. It's not stuck, it just takes a LONG time if there is a software download, which there will be if your machine shipped with the older (e) software. I checked my Vonage account and saw that it was connected to the New York Tivo number above for 6 1/2 hours!!! Nevertheless this morning I noticed that the call was successful and I had the newest (f) software. For this reason I recommend forcing the daily call right before you go to bed. Just make sure it connects and starts the download.
I forced another daily call tonight to see how long it would take and it only took the usual 1-2 minutes. If this above method doesn't work for you try some other methods in the Vonage compatibility thread in the Tivocommunity forums.