I'm tring to read my Landis+Gyr ZMD meter via GSM modem. It work fine! Very good job! But I have found a problem in GXTerminal library.
When it close the connection the modem don't hangup.
It's true thet we are not expecting reply at +++ but "OK" reply may be if the modem is in data mode, and in this state you must wait some second before you send ATH0 command.
To debug them I'm used your "GXTerminal csharp Sample" but obviously the same problem is present in GXDLMSDirector
To workarround them I have inserted a 3 second Thread.Sleep before the SendCommand ATH0, and all work fine!