If you hold your iPhone in your left hand to make calls, you might want to reconsider.
A study has shown that the way you hold the phone can affect signal strength, and that it becomes weaker when held in the left.
Look at this table, via Quartz:
The results come from the Nordic Council of Ministers, investigating how well different phones picked up and transmitted radio signals.
Apple’s new iPhone 7, which was released this week, wasn’t part of the study.
Maybe in that place where the headphone jack used to be, they might have sorted it out?
Otherwise, just stick to the hands free maybe.