>I'm trying to write a VB app that establishes a modem call (using an internal PC modem) for simple COM port serial communications (not dial-up networking). Once the call is established I want to pass the established comms link to another application program (a program which uses simple serial COM port traffic but is not modem-aware). It appears that when my front-end program drops the COM port to the internal modem in order to launch the target application the call is automatically cleared - presumably Windows detects that, for a brief moment, no application program is connected to that COM port and forces the modem to clear down.
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>Does anyone know how to prevent Windows from taking control of the modem. I have tried instructing the modem (via AT commands) to ignore DTR (assume ON), but it seems to make no difference, although this technique does work for an external modem.

Persist the owning app and then share the handle to the modem session is the only thing, short of creating a COM component to virtualize the modem interface.