By default a guest machine in VirtualBox is configured to use so called NAT networking - the guest's network adapter's IP is in a "virtual" subnet which includes the guest and the parent VirtualBox application which acts as a gateway. For the guest it looks like it accessing internet via something like an ordinary DSL modem, but the guest can't be accessed from anywhere, just like you can't access machines in your local network from Internet without specifically configuring your modem to allow such access.

VirtualBox documentation has the following list of networking modes:

Each of the eight networking adapters can be separately configured to
operate in one of the following modes:

Not attached

In this mode, VirtualBox reports to the guest that a network card is
present, but that there is no connection -- as if no Ethernet cable
was plugged into the card. This way it is possible to "pull" the
virtual Ethernet cable and disrupt the connection, which can be useful
to inform a guest operating system that no network connection is
available and enforce a reconfiguration.

Network Address Translation (NAT)

If all you want is to browse the Web, download files and view e-mail
inside the guest, then this default mode should be sufficient for you,
and you can safely skip the rest of this section. Please note that
there are certain limitations when using Windows file sharing (see the
section called “NAT limitations” for details).

Bridged networking

This is for more advanced networking needs such as network simulations
and running servers in a guest. When enabled, VirtualBox connects to
one of your installed network cards and exchanges network packets
directly, circumventing your host operating system's network stack.

Internal networking

This can be used to create a different kind of software-based network
which is visible to selected virtual machines, but not to applications
running on the host or to the outside world.

Host-only networking

This can be used to create a network containing the host and a set of
virtual machines, without the need for the host's physical network
interface. Instead, a virtual network interface (similar to a loopback
interface) is created on the host, providing connectivity among
virtual machines and the host.

Generic networking

Rarely used modes share the same generic network interface, by
allowing the user to select a driver which can be included with
VirtualBox or be distributed in an extension pack.

If you need to access your guest from outside, you need to configure bridged networking, which will give your guest its own IP in your local network. The configuration is done in VirtualBox settings, not in the guest OS.

To be able to access the server from your mobile (i.e. from outside of your LAN), after configuring the networking you additionally will need to set up port forwarding on your DSL modem.