Be sure that sound cards, mice, and other hardware don't use the same interrupt as your modem. Try removing CD-ROM drivers, if possible. A lot of complaints come from people using the DOS that comes with Win95. If you are using this version of DOS try getting a DOS 6.22 boot disk and see if that will help. Kali puts a load on your modem that is quite different from most applications.
Most applications only receive data at full speed.
Usually these lockups occurs because the communication between the PC and the modem breaks down and the PC gets stuck trying to send packets. You might be able to get Kali into an upper memory block on some systems. Using DOS 6.0 and above you can create multiple configurations in your config.sys. They also still need at least some minimal conventional memory. Upgrading from a 14.4 to a 28.8 modem makes a world of difference in almost all cases.