Modem E1552 - No voice with VB Application

Hi,

In my system, I need to use this modem in this two mode : Data and Voice.
For the Data part, no problems.
But for the voice part, I have no sound.
To call someone, I use AT commands in a VB.NET application.
The communication are OK. I receive or i can send a call, but no sound pass beetween the two parts.
Of course, with AT commands, I change the sound and don't put in HP mode :
AT+CMUT = 0
AT+CLVL = 4
If I use Mobile Partner application, The voice are up. If I compare the AT log with mine, the two applications send the same commands.
I think the voice data are not transmit to the audio systems(HP, microphone)

Excuse me for my bad english.

Thanks to help me !