First thing's first, make sure that your line is capable of 20Mbps. You can hop over to our site and talk to an agent if you'd like us to check for you?
Secondly, is this 20Mbps on VDSL or ADSL? If only 20Mbps then I suspect you're connected to an IMAX which isn't capable of VDSL, which requires an MSAN connection.