I am moving into a new home and would like to get a cable modem,
directv with tivo (older box that I already have) and vonage for my
phone service. I understand that tivo can not make it's daily calls
over a voip line. Is this true? What are my options for using the
directv/tivo box without getting a phone line.

Tivo usually can make its daily call over VOIP...the trick is to slow
down the baud rate enough to make the data transfer reliable.

On many of the Tivo boxes you can force the baud rate by going into
Messages & Setup, Settings, Phone, Change Dialing Options, Set Dial
Prefix.  Then make the prefix #019 -- this will force the baud rate to
19.2kbps.  Some have also had success with prefix of #096 which is the
9600 baud rate or #034 for 33.6kbps.  Note that you may have to
precede the # symbol with one or two commas (,) to create a slight
delay before dialing.  Also some have found that they must include the
1 and area code as part of the dialing prefix too.  So an example
using that format would be (remove the quotes) ",019,1xxx" (where xxx
is your area code).

The problem of getting Tivos to work over Vonage has been around a
while and there are many that have gone before you.  There are also a
lot of different ways that people have gotten it to work.  Some opt
for a USB ethernet hack that has been well documented.  Some have made
changes in their Vonage setup to favor "quality" over speed.  Some
have spoken with the right techs at Vonage who apparently can get you
up and running.  You can also use external modems that allow greater
control of the parameters.  You'll find plenty of descriptions of what
has worked for others at www.tivocommunity.com, www.vonage-forum.com,
www.dealdatabase.com/forum, wiki.ehow.com...but you may find the very
basics that I shared with get you up and running.