Every modem will be compatible with someone, but we have yet to see the modem that is compatible with everyone. Our systems have been very reliable with current versions of firmware on modems from Supra, US Robotics, Motorola and many other brands. Our biggest support headaches have come from the very low cost modem units. As modems become faster, they have also become more complex. Make sure your modem register settings do not restrict its ability to adapt to phone line conditions.
Some modems have a way to record the reason why a connection failed.
If you can, try using an external version of another brand modem and see if the problems disappear. This software will interfere with a PPP connection to the internet. Another common cause of disconnects is line noise. Also unplug your laser printer from the AC wall outlet. The modem should be the ONLY thing connected to your phone lines.