I assume Canadian regulations are similar to those in the US. Regulations specify voice quality. Modems take advantage of the fact that in most cases loop quality is much higher then required allowing to modem to operate faster then if worst case line conditions are assumed.
1) How is voice quality? Any noise on line, especially when phone is first picked up? What about crosstalk, other voices? If voice quality is not up to snuff have the phone company repair the line.
2) I doubt the ISP modem is at fault. Assuming your ISP supports V.90/92 ISP side is directly terminated to T-1 or other digital service. V.90/92 requires that one and only one digital to analog conversion exist between ISP and customer. That normally occurs at the central office serving the customer.
Most likely your phone company is using some form of pair-gain equipment to multiplex multiple voice customers over a single circuit. This can significantly degrade modem speed.
I've posted more info on my site if you are interested in the gory details.
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