23.0403GPPRelease 17Technical realization of the Short Message Service (SMS)TS
As the consequence of the replying procedures specified in this annex, all SMEs and applications within them may assume that replying from all mobile stations is always possible, provided that the mobile stations do support the proper replying mechanism itself (human response in context with the original MT SM, automatic replying by an application, application level protocols, etc.).
Annex E (normative):
Extended Object Format Type