voip.ms service ----> modem ----> router ----> Obi100 ----> phone line.
Is Your Line VOIP Ready?
First test your internet line for "voip readiness" using an online tool like this one. Just let the test do its thing... It will measure download/upload speeds, line jitter, packet loss and something called MOS score. If you get 4 green dots then you're good!
If your results aren't all green then try plugging in your laptop directly to the router and do the test again. If it still isn't good, make sure the modem is as close as possible to the line coming in and the cable between the modem and router is of good quality and not longer than a 50 metre run. If none of this helps, then contact your service provider and ask them to fix your line.
Buying the Obi and Porting Your Number
Once you are confident your line is ready for voip, you are ready to purchase an Obi device... any one will do, the Obi100 is the cheapest and gives you a complete phone line. The other models do more and might serve your specific situation, so look into them as well. I purchased ours on amazon and I think they've come down in price since then. Also sign up for voip.ms. You can keep your existing phone number by doing a porting process. All the instructions for porting your number are here.
Get the Obi device before the date of your scheduled phone number port. There are a few feature you can try out beforehand. On the day the number is ported, follow these instructions to get your phone line working.
Later on I'd recommend to follow mango's excellent guide to tweaking your Obi device.