Before getting too complicated, might be worth checking the easy stuff first - telephone plugs & sockets around the house. Wiggle the wires and see if your router or telephone disconnects. Pick up the phone and see if the line is crackle free, or if you lose the internet connection, pick up as well to confirm there isn't a line fault.
This happened to me a few weeks ago, assumed it was an internet problem for ages until we noticed the phone momentarily dropped the connection at the same time as we lost the internet. Got up a ladder and traced the fault to a broken wire in the telephone line from my house to the telegraph pole. Strangely this was caused by a tree growing round the cable and gradually crushing it over a number of years! One new cable - problem solved.