I've never had a mobile phone slip out of my center jersey pocket. I guess I have been lucky. Might be different if I was riding a mtn bike on trails or a gravel bike on rougher roads. I keep my key fob, credit card, bus pass, and a little cash in an under the seat bike bag along with tubes and tools. After reading this I think I might try using a big thick rubberband as was mentioned earlier. Seems simple enough and a little more secure.