How do you know that they are compatible ?

How did you get the speed ? if the router is still in warranty period, you should get a replacement, I think. Have you contacted the retailer where you bought it ?

The brand of your VOIP device ? The model number ?

Hey, I found a troubleshooting guide here: http://www.tp-link.com/en/article/?id=400

They are plugged into the wall sockets in the same electrical circuit, right ? Your computer connected to one of the TL-PA211 with a lan cable, right ? If yes, try another computer and try another...

Wow... In the end of the flat, your PC connected to the extender can access the internet now ? How did you install extender ? And the distance between the root router and the extender ? and between...