Describe the bug

write_call_cp looks up the appropriate TBS instance when operations are done on GTBS. In those cases, we correctly take the mutex of GTBS, but we never take the mutex of the TBS being looked up, e.g.

		call_index = ccp->accept.call_index;

		if (is_gtbs) {
			tbs = lookup_inst_by_call_index(call_index);
			if (tbs == NULL) {
				status = BT_TBS_RESULT_CODE_INVALID_CALL_INDEX;
				break;
			}
		} else {
			tbs = inst;
		}

		status = accept_call(tbs, &ccp->accept);
		break;

This means that the call may be changed by a different thread while we are handling it here.

Regression

• This is a regression.

Steps to reproduce

N/A

Relevant log output

N/A

Impact

Functional Limitation – Some features not working as expected, but system usable.

Environment

28e25ea

Additional Context

Relates to #104655