Microtech and Microcom for two of them. USR is also good, but very pricy. Microcom is about the best as far as I can tell, but not as easy to get as a USR modem. I have an old Microcom 28.8K BPS modem that was upgraded to 33.6BPS V.34 with just a single chip. This was back in 1996, but I assume they still make hardware modems. Mine is an external, but I lost the power supply to it in a move in 1998. But it always was reliable, and cost effective and upgradable. MNP Level 10 as well, something that other modems did not have at that time, they only had MNP Level 5.