Comment 6 for bug 1617961

It turns out that implementing a UI for this properly is a bit complex. This is the standard way that SMS works on all smartphones based on the characters in the message.

In short several things need to be observed and accounted for:
Any character not in the GSM alphabet causes use of UCS-2 (certain escape chars, emojis)
Any extension GSM character requires an extra character. Any such escape character must have both parts in the same message and can therefore leave 1 more unused char.
Multi-part messages introduce header overhead in each message in both GSM (7 chars) and UCS-2 (3 chars)