These settings have been known to work with VOIP-connected TiVos (YMMV).

Set Dial-In Number: (local #)
Set Dial Prefix: 2122773895
Set Call Waiting Prefix: ,#034,*99,
Set Tone/Pulse: Tone
Set Phone Avail. Detection: Off
Set Dial Tone Detection: Off
Other:
Dial Prefix: ,#019 (uses 19.9kbps)
Set Call Waiting Prefix: *70 Disables call waiting