Your RAS server never answers at 56Kbps.
Your connections are at 33.6, 28.8, or slower.
Normal analog phones lines do not support 56Kbps standards. To consistently connect at 56Kbps, you need a digital line and special hardware. You must meet the following criteria:
1. A digital circuit is required at one end, with a digital modem.
2. Both modems must support V.90.
3. No more than one analog to digital conversion is allowed in the link between the two modems.
See the 3Com white paper at http://www.3com.com/technology/tech_net/white_papers/500659.html.