I'm trying to connect to my phone from my Windows 7 PC using PySerial with the following code:

import wmi
import serial

c = wmi.WMI()
modem = c.query("SELECT * FROM Win32_POTSModem").pop()
ser = serial.Serial(modem.AttachedTo, modem.MaxBaudRateToSerialPort)

try:
    ser.write('at \r\n')
    print ser.readline()
finally:
    ser.close()

But get the following error on the write call:

Traceback (most recent call last):
  File "D:\Alasdair\Documents\Python Scripts\Phone Interface\test.py", line 14, in <module>
    ser.write('at \r\n')
  File "C:\Python26\Lib\site-packages\serial\serialwin32.py", line 255, in write
    raise SerialException("WriteFile failed (%s)" % ctypes.WinError())
SerialException: WriteFile failed ([Error 6] The handle is invalid.)

I've tried connecting with TeraTerm and that works fine, so it's not a problem with the connection to the phone itself.

I've been searching around for ages trying to find a solution but haven't come up with anything that works. Any ideas?