Handle change-of-network gracefully

Pjsua is unable to handle a change of network (i.e. where your externally-facing IP changes) properly. Sending/receiving texts and making/accepting calls doesn't work. Critically, it doesn't even let you know something is wrong - it looks like texts are sent and calls are attempted but nothing actually happens. Note that this doesn't affect simply losing connection and re-establishing on the same IP.

We need to do the following:

1. Have some kind of way to know when the connection is broken (so the user knows that e.g. their calls aren't going through)
2. Implement this so that we can handle the change of IP properly