telnet 127.0.0.1 [HOST_PORT]
The "catch" is to use 127.0.0.1 and not guest's IP (i.e. 10.0.2.15).
If everything is working properly, VirtualBox port forwarding will forward you to appropriate guest's port.
Example of setting port forwarding:
Also, it's sometimes a good idea (at least in testing phase) to leave both Host IP and Guest IP blank. That way, no IP filtering will be done on any end.