The voice service on your phone line is separate from your broadband one. I've had a phone line which didn't work at all with phone calls but still gave me an excellent broadband service. Equally it's perfectly possible (and, indeed, far more likely) to have a phone line which is perfect for calls but has a pathetically slow broadband speed.
If you're getting a much lower broadband speed than you'd normally expect (with a wired connection between your router and your computer) and
(a) you're sure that there's not something else using your internet access (such as an internet-based TV service or, if there's no password on your wifi, a neighbour 'piggy-backing' your access) ;
(b) you've given it a day or two to see if things get better automatically;
you need to speak to your ISP to get them to contact BT OpenReach.