[lxgr]

I've been wondering about that! A long time ago, I had a phone plan like that as well (in fact, a public IPv4 was the only option), and I was wondering if random port scanning would eventually use up all of my data allowance.

Turns out stateful firewalls aren't just a security factor on mobile internet connections.

[korhojoa]

I've luckily mostly been on unlimited data plans, so I don't know about it using up your data, but I guess it would? Your device is sending and receiving data. I don't think there's a distinction for externally made connections, otherwise vpn tunnels would have been really popular.

I did have an experience with my phone being suddenly used by random internet users. I had ampache running on my n900 (maemo/debian-based) and somebody found it on the internet and started streaming my music. :D I had a higher-than-normal amount of battery usage for a few days so I decided to see what's using it and discovered that my music was being played and downloaded.