A fire alarm is a loud and shrilling noise, so the simile is emphasizing how loud and shrilling the cell phone went off.
Context: His hand slid up, fingertips brushing against lace and- his cell phone went off in his pocket loud and shrilling
Hypothesis: His hand slid up, fingertips brushing against lace and- his cell phone went off in his pocket like a fire alarm.

Given this context, is the hypothesis true (Yes/No/I don't know)?