Having that problem too but it's indoor so nothing to comment about but the weird thing is it only happens in my Engineering Lab...at the Cafeteria (lower floor) near max signal.. so I also ??? liao.

You try to get another phone and spend some time test calling with a friend.
Sometimes signal bar cannot be trusted. Either you get a dB app to monitor or that.

Anyways if u still got this problem and it really disrupts your calls, jot down the location, go somewhere else, test.

2nd test is, get another handphone, Nokia or anything, slot in SIM, check signal. Make sure it's not your Telco instead of waste time going to SIS for something that is beyond their jurisdiction.

Hope all goes well.

Never root can send anytime no problem but for me, I don't like buang masa go all the way there just to repair something inferior-ish.