Dial-up connection dropped within few minutes

Hello,

I'm building an XPE image with a USB voice modem. When I attempt a dial-up connection on it, it works properly for a little and
variable time, then the connection is dropped. In the modem log, I get
- CD dropped--Remote modem hung up. ModemStatus=00000030
A change in the S10 register (loss of Carrier Detect timeout) of the modem does not affect the disconnection.

Moreover, the same modem with the same driver on XPPro work fine (i.e. the connection is never dropped).
A dial up connection on the same target with a serial modem works fine, even when the serial modem is connected to the USB port
through a USB-to-serial adapter.
The Ethernet connection on the same target works fine.

This same error happens with different ISPs.

What can cause this?

Thanks
Anna Trompetto