Cannot get clean sound from ZTE voice modem

My problem is associated with ZTE usb data modem.

ZTE USB modem have Communications(COM1), Diagnostics(COM2), Service(COM3) and Voice(COM开发者_开发百科4) ports. I can receive and send voice to voice port. But i can not get clean sound like WCDMA UI. How can I receive and send data with high quality?

Please look at my Visual C++ source code: http://serv7.boxca.com/files/0/z9g2d59a8rtw6n/ModemDial.zip

Does anyone now where is my error?