A couple of years ago, I've used my TTY with Vonage and I get no garbled text. Easy thing to do is set your VoIP device between your router/computer and cable/DSL modem, but only if a device has a LAN port for that to work.
Here's what I mean:
[Internet] <--> [Cable/DSL modem] LAN <--> WAN [VoIP Device] LAN <--> WAN [Router (if you have one] LAN <--> WAN [Computer]
If you have a VoIP device behind your router, be sure you set your Quality of Service (QoS) to High for either an Ethernet port or a MAC address (12 character hexidecimal from 0 to 9 and A to F). Please refer to the manual for your router.
Other than what I've mentioned about having VoIP device behind your router, just have the VoIP device between your router and cable/DSL modem. But remember to refer to the manual that came with the device.