My mobile Internet provider (tele2.se) blocks port 22 for incoming traffic. You can check your status with this web page:

http://canyouseeme.org

Just look up the page from your phone when connected via 3G and enter the port number (this page will also show your IP address).

So I changed to port 24 (they only blocks 22, 23, 110, 1080, 1433, 3389, 5900, 8080). In the file /etc/ssh/sshd_config, change the line "Port 22" to "Port 24" or the port of your choice. I did this and the abovementioned web page can now see me. And more importantly, it works.

But be aware of the security risks if you succeed!