You can not use a telephone cable as short as wall outlets are located in places near sources of heat, angles, etc. and look at our house you can not set the computer near wall outlets. In addition, we would have wires all over the place, making great opportunities to travel and get hurt, or put together tables.
Phone wires travel from the Central Office, to (and through) your home as a twisted pair. The pair of wires is twisted to cancel cross-talk (foreign noise induced on the pair). The quality of your connection is dependent on a SNR(signal-to-noise ratio). Using a long line-cord is not the best idea, but if you have no issues, give it a try. I would get a baseline SNR near the jack, and compare it to the SNR on the long line-cord. It would be in the modem transciever statisitcs under SNR, or Noise Margin. Make sure you are comparing apples for apples though... make sure your downstream and upstream data rateson the second test match the rates on the second!
When we talk about length limits for DSL, there is the twisted pair from the Central Office to the modem, and the two or more twisted pairs from the modem to the PC. 25000ft is the untreated theoretical length limit to the modem. The 300ft limit would be for Ethernet.