>Hi,
> I would like to write an application that will control modem. However, it seem the command set of each model of modem may be different. Therefore, I plan to do this by calling the device driver. Is it possible for VFP application? How to do so? Any reference?

You could check out ActiveX component providers to see if any are available with the functions you want e.g.

http://www.componentsource.com

Failing that, you could investigate the Telephony API (TAPI) and see if that can do what you want. My understanding is that it abstracts telephony functions and isolates you from having to deal with device drivers directly.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tapi/tapiovr1_7juf.asp?frame=true

Even if that works for you, VFP may not be the best choice for low-level work like this - you might be better off with VB or VC++.