And just to build on this - there was a good reason to. If you're hosting one of these behind a domestic network connection (in the UK, at least), there's a very high likelihood that you're using a modem that will let you port forward, and port trigger, but not at the same time. We decided the best workaround was to set dropbear to listen on a non-standard port, so we can implement straight port forwarding only on the modem side (there's also a slightly reduced chance of script kiddies and Shodan showing up).
We saw in /etc/init.d/dropbear that it starts up dropbear as a daemon, but adding the -p XXXX flag to specify a port number was just ignored. A little duckduckgoing suggested dropbear can listen on alternative (and multiple) ports, but couldn't find a huge amount of relevant information. Quite possibly, I'm doing something stupid and am just not familiar enough with the distribution running on the Kerlink box, but I suspect that this is something that will come up again, hence the query here!