How to delete VirtualBox networking rule

The solution was pretty simple: I just had to add the following lines in Debian virtual machine's /etc/network/interfaces file:

allow-hotplug eth1
iface eth1 inet dhcp

The second line instructs the interface to obtain an IP via DHCP. The first line loads the interface at boot time.

To apply the changes to a running system, invoke:

ifup eth1

The name for the eth1 interface may vary, use ifconfig -a to list all available interfaces.

EDIT: full /etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

allow-hotplug eth1
iface eth1 inet dhcp

Trying to use PCI pass-through on VirtualBox is dicey, it's still an experimental feature. Your hardware needs to support IOMMU which is uncommon in consumer-grade equipment.

The host OS needs to recognize the device and create a serial port for it, /dev/ttyS? or /dev/ttyUSB? or any of a series of other options depending on the distribution.

If it does, use VirtualBox's support for serial port pass-through and tie the virtual device, eg. COM1 to a Host Device and supply the appropriate device path (like /dev/ttyS0) in the text field.

When you restart the virtual machine, you should be able to access the modem from the appropriate /dev/ttySx port starting from 0 = "COM1"