af91c418_r_Broadband_Speed___The_Basics__an_affect_your_broadband_speed
[Cause] Telephone Faults
Broadband is supplied over your telephone line, so it needs the line to be working properly. Subsequently, it's common for telephone faults to cause problems with broadband service. You may see slow speeds or you might not be able to connect at all. In nearly all cases, once the telephone line is fixed the broadband will be too. Connection problems tend to clear up immediately, with slow speeds it'll be a few days before your speed gets back to normal.
Your Broadband Speed - The Basics
How this can affect your broadband speed