Sending SMS containing emoji fails

Seen on a PinePhone on both pmOS and Mobian.

Sending any message with an emoji immediately fails (cross mark next to message, recipient doesn't receive).
Inbound SMS with emoji work fine.
Sending a message with a character like ® (not part of the GSM 7-bit alphabet according to https://en.wikipedia.org/wiki/GSM_03.38) works fine.

Sending emoji shouldn't require MMS support (almost certain it doesn't), which I know is currently not implemented per #30 (closed)). Is this to do with the encoding currently being used to send SMS (see https://en.wikipedia.org/wiki/GSM_03.38)?

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