- Make `putchar` faster (probably just `stamp_char`) because it is currently
  too slow to keep the modem at a high speed without dropping data.  We may
  be able to just get away with using flow control here if we can't make things
  fast enough.

- Add an actual Settings menu to change modem speed through DLAB, speaker
  volume from automated calls, etc.  This will require persisting settings to a
  dataflash page somewhere.

- Add a Call menu to dial saved phone numbers.

- Maybe add battery level to status bar?