Many 56K modems are not v.90. If you are NOT connecting in the 40's to 50's then go to your modem manufacturers web page and see if they have a v.90 update.

The following sites contain good info about 56K.

To test to see if your phone lines will support V.90 click here.

Lots of good info can be found at 56k = v.Unreliable

You can also check out for more info on the V90 Standard.