Gsm Redialing Problem

Hi to all!

I'm having a problem while trying to make gsm calls with my Westermo GDW-11 Modem. After making the first call to a number, i'm trying to make a call to another phone number. But i can't do it, because i can't hang up correctly. My DTR Line is always connected, even after using SerialPort dispose, setting DTREnable to false... and sending +++ and ATH commands to it... Anyone could help me with this? Is there any kind of documentation regarding the good way of making more than one gsm call sequentialy? My project deadline is approaching and this point is crutial.

Best regards.