If you plugged a telephone into your D-link router, unplug it - that can not work!
You have to plug your vonage router (WAN port) into the D-Link (LAN port) and connect your phone to one of the LAN ports of the vonage router.
If you do not need the wireless capabilities of the D-Link, you can simply take that one out and only use your vonage router.
If you need more help, I would need more details form you:
Which vonage adapter are you using?
How is your setup (i.e. modem -> D-link -> vonage & computers)
What connections are you using (wire/wireless)?
You can also find help
here.