Phases 1 2 3 have a CONSTANT duration, which do not depend on the size of the SMS. Whether the SMS is 1 character or 300 characters, those phases 1 2 3 are always lasting the same, very short, time. Less than 0.5s, I believe.
Phase 4 has a duration which depends on the SMS size. If sms is 1 character long, then it will be very short. If sms is 300 character long, then it will take much more time.
Those are the facts.
Now, I added some ideas below, in earlier posts, that you may investigate. We can also wait for other feedbacks on your results.