The speed is quite adequate. As a comparison I have no problems at all in streaming a YouTube video without buffering on a 1 Mbps 4G mobile connection (it's a restricted speed mobile package I have because it's cheap for when I'm out and about) the video uses much more bandwidth than audio on a VoIP so you're safe. Also I have used Vonage to call the UK on that 4G connection and I didn't even have any latency problems. It's about whether the connection is reliable in terms of uptime. I don't have any problem with fiber but if it's a copper telephone line I have had issues in the past with faulty connections and rain even. You won't mind if your connection goes down momentarily if you are just browsing on the web but the last thing you want on a business call is for the call to drop. But you won't really know how stable the connection is until you try it.