Welcome to the forum @Stel67. It isn't normal that you have been getting these issues. What I would like to recommend is to start by resetting the modem. I would also like to know if there are any appliances other than the regular mobile phones and TV modems in use. Sometimes these interruptions are caused by things like personal routers or settings that have been changed in the Experiabox. 

Thanks Bram for your answer 

I received a new router from KPN and the problem solved. In the meantime I replaced the sockets and part of cables. Probably the problem was the router but also the home network is always essential for quality connection.