This phone uses serial-over-USB. From the motorola developers forum:
We do not normally support the use of non-PC RS232 connections to current Motorola handsets. The only serial connection supported is serial-over-USB, where the handset is connected to a USB port on a PC, and the PC operating system assigns a virtual COM port to the USB connection for this purpose. Microsoft Windows is normally needed to meet this requirement.
For example moto4lin communicates successfully with this phone using AT commands.