In order to change the default SSH port in the FOX board, I changed the following line in the init script /etc/init.d/ssh:
#start_daemon /usr/sbin/dropbear $SSH_OPTIONS
start_daemon /usr/sbin/dropbear -p 3333
In theory, it should be enough just to change the $SSH_OPTIONS set in the configuration file /etc/conf.d/ssh. However, I tried it, but it was not changing the port, so I used the above solution (changing the init script).