The original ETSI TS 100 900 V7.2.0 (1999-07) Digital cellular telecommunications system (Phase 2+);
Alphabets and language-specific information
(GSM 03.38 version 7.2.0 Release 1998) defined byte 0x09
as Ç
(capital C with cedilla).
Subsequently in GSM 03.38 to Unicode mappings, a clarification was made:
General notes:
This table contains the data the Unicode Consortium has on how ETSI GSM 03.38 7-bit default alphabet characters map into Unicode. This mapping is based on ETSI TS 100 900 V7.2.0 (1999-07), with a correction of 0x09 to small c-cedilla, instead of capital C-cedilla.
and in the table:
0x08 0x00F2 # LATIN SMALL LETTER O WITH GRAVE
0x09 0x00E7 # LATIN SMALL LETTER C WITH CEDILLA
#0x09 0x00C7 # LATIN CAPITAL LETTER C WITH CEDILLA (see note above)
0x0A 0x000A # LINE FEED
So there you have it, this character was remapped at some point. It is likely that you are correctly-encoding the character, but an older device or something using a library with the old standard is interpreting the character according to the original mapping, resulting in the capital letter.
I'm not seeing a mapping for Ç so it shouldn't appear any more.