First Not all 56K modems will work at 56K on all Internet Service Providers. There were dozens of different manufacturers making modems for both 56K modem types.
Some manufacturers even make modems for both types.
All of these manufacturers are now making the V.90 modems. Many modern computer manufacturers are including poor quality modems in their systems that are software controlled. They are subject to problems such as frequent disconnects, slow connection speeds, and the inability to get connected at all. You may also purchase any modem that states it is upgradable to V.90. As a matter of fact, there are still many very good 33.6 and 56K modems being sold at very cheap prices right now that can be upgraded to the new V.90 protocol.