Modem speed

I have a delphi program which uses modem component to connect from PC
side modem to a device side modem. I find that the program does not
work with some kind of modems. (Some other kind of modems works very
stable!)

After connection my program will change the connection speed 19200
down to 9600. However, I find that when I do this, the connection
terminates.

Do anyone know why some kinds of modem will terminate the connection
when I change speed and how to solve it?

Because I cannot control which modem my client will use so that my
program needs to work with different kinds of modems!

Thanks!