This is data sent by modem via serial port. As you see - initially it looks normal and there are is AT communication. Modem is working in 'command mode'.
After connection is initialized - your modem enters 'data mode'.
wvdial cannot recognize when modem switches to 'data mode' and prints everything that came to serial port.
This is quite common in USB cellular modems. These modems are not standarized.
More information:
Command and Data_modes (modem) on Wikipedia - about modem modes
Hayes command set on Wikipedia - how Hayes-compatibile and other modems indicate mode switching