You may find that the signal levels are not high enough to generate a maximum speed connection. That's usually not a major disadvantage, provided that the signal is clean. My own connection was at a consistent low level but also consistently clean. It is possible to test line quality with some modems. It is possible to use the data to back up a complaint on voice quality issues.
Most common problems involve static and high frequency dropoff.
I often have a difficult time getting people to hear the number 'six' in conversation. My phone lines show a frequency degradation at the high end, and I suspect that if my line had better high frequency response, others would hear me say 'six'. To test line noise with the modem, find a known good connection, such as an internet provider with ISDN lines, digital lines, or 56K capable lines. Some of these tests require that you escape to command mode with the escape code. They give you a nice graphic representation of your signal response.