A peer to peer phone relay network built on top of yggdrasil.

Every box is reachable from any yggdrasil node running this service, not only
from members of the same clan.

## Numbers

Each box gets a random 6 digit number (100000-999999) when its vars are
generated. The number is signed with the box's yggdrasil private key, so a
number is cryptographically bound to the yggdrasil address it belongs to;
nobody can claim a number for an address they do not own. Regenerate the
`phonebox` vars to get a new number.

Dialing from a phone plugged into the ATA:

| dialed        | reaches                                    |
| ------------- | ------------------------------------------ |
| `NNNNNN`      | the phone on box `NNNNNN` (line `00`)      |
| `NNNNNNXX`    | line `XX` on box `NNNNNN`                  |
| `00`, `01`... | local lines on this box                    |
| `888`         | fax of the current number directory        |
| `000`         | fax echo test                              |
| `999`         | hello world                                |

Caller ID on the callee is the caller's box number followed by its line, so
calling it back works.

## Directory

There is no central registry. Each box runs a serf agent on yggdrasil port
7946 that gossips its signed number record (`number`, `key`, `sig`, `owner`
tags) to every other box it knows about. On every membership change the
`phonebox-sync` handler verifies all records and writes the resulting
`number -> yggdrasil address` map to `/run/phonebox/numbers/`, which the
asterisk dialplan reads at call time.

Bootstrapping: boxes of the same clan join each other automatically. To
connect to boxes outside the clan add one of their yggdrasil addresses to
`extraPeers`; after the first successful join the agent remembers the whole
membership in `/var/lib/phonebox/serf.snapshot` and rejoins on its own.

`serf members -rpc-addr 127.0.0.1:7373` shows the live directory.