var
hCommFile : THandle;
.
.
.

procedure TForm1.Button1Click(Sender: TObject);
var
PhoneNumber : string;
CommPort : string;
nw : Cardinal;
begin
PhoneNumber := 'ATDT 0603123456' + #13 + #10;
CommPort := 'COM3';
hCommFile := CreateFile(PChar(CommPort), GENERIC_WRITE, 0, nil, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
if hCommFile=INVALID_HANDLE_VALUE then
begin
ShowMessage('Nelze otevřít '+ CommPort);
exit;
end;
nw := 0;
if not WriteFile(hCommFile, PChar(PhoneNumber)^, Length(PhoneNumber), nw, nil) then ShowMessage('Nelze komunikovat přes ' + CommPort);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
CloseHandle(hCommFile);
end;