The raw speed is not the issue. The issue is that VoIP phone calls rely on an uninterrupted, good quality connection.
When you watch a video online, it is nearly all downstream data (from the video server to your computer), and the data is buffered on your computer (like filling up a water tank, and then draining it out while refilling it, which can hide temporary loss or delays of transmission). VoIP can't be buffered more than about a quarter of a second's worth, before it would drive you crazy, talking over each other.
DSL is asymmetric. That means, while you may get something under 5Mbps (it's never as fast as advertised) downstream speed, the upstream speed is likely much lower, like 256Kbs, or 1Mbps. With other computers in use, if someone is doing gaming or videoconferencing, the upstream bandwidth will be sucked dry, and your outbound part of the call (you talking to the other party) will break up or be severely distorted. I don't recommend that sort of connection for a multi-user office, but you can certainly try it yourself and upgrade your service if necessary.