Converting UTF-8 strings to hex in Emacs

[白い熊]

Hi:

I'm looking for the Emacs way to convert UTF-8 strings to Hex, so they
can be sent as hex codes to modem AT commands.

For interactive mode hexl-mode lets you view strings and see their
representation, however I need a way to do this in elisp, i.e. pass the
UTF-8 string and receive back its hex representation.

There's uni2ascii which does this, and I can call it via a call-process,
however would like to do it in elisp.

Anyone know how?
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白い熊