May I suggest. When experiencing these symptoms. Restart your router. If the problem persists albeit intermittently your router may have developed an intermittent fault. Check / replace Microfilters (sometimes known as ADSL filter) Check any other phones on extensions by disconnecting them and see if the problem persists, they also should be connected through a microfilter one of them may have developed a fault. Contact your service provider for replacement and or check your line for noise. A noisy line will cause intermittent ADSL failure.
Your service provider will make you go through the above before either replacing your equipment or sending an engineer to diagnose fault on site.
If you want a shortcut to a replacement router, tell them your router has failed. i.e. you have tested it in another power socket and on another phone socket and there are no lights at all showing on the router. They will normally send a replacement very quickly.