With regards to what bill001g said about 8.8.8.8. , you could also try the following.
right click the icon to the internet to the left of your speaker icon , click on change adaptor options , right click your internet connection icon and choose properties , click on internet protocol v 4 to highlight it then properties , on the bottom half of the page you are looking at now , click on the button use the following dns , now in the first box you need 8 8 8 8 and in the second you need 8 8 4 4 , finally tick validate settings before exit .
How many inline filters do you have on your phones or other pieces of equipment , and also how old are they.
Sometimes a filter can go faulty without you knowing and this can effect your set up. The only way to test if one is faulty is to get a spare one and try it on each phone etc that you have and see if you still have problems.