Mixed text do not exist, or is CP1252 or is CP1251 (or another)
UTF-8 have all symbols.
The PDU can be encoded GSM-7 (or ascii-8) or UTF-2, not any CPx.
If Cyrillic is present on SMS, is sended by UTF-2, not GSM-7 (or CP1251)
Function to convert UTF-8 to UCS-2
http://forum.mikrotik.com/t/convert-any-text-to-unicode/164329/26
Function to convert UTF-8 to UCS-2 string for create the mesage part on SMS PDU encoded with UCS-2.
http://forum.mikrotik.com/t/convert-any-text-to-unicode/164329/26
Function to convert UCS-2 to UTF-8
http://forum.mikrotik.com/t/convert-any-text-to-unicode/164329/18