Is there an issue with Davening from a cell phone app?
When Davening it is appropriate to use a Siddur. Besides for the Kedusha that the Siddur has, which the phone does not, the phone is likely to generate unwanted disturbances which are not appropriate during Davening (messages, etc). Chazal tell us that one must clear his thoughts from all worldly dealings before Davening to Hashem – this is definitely not enhanced by Davening with a phone, especially if the other apps are not switched off and the phone isn’t on airplane mode.
Another issue is the Issur to hold expensive items while Davening, as the concern that it might fall distracts from one’s Kavanah. In general, during Davening one must stand in a state of seriousness as one stands in the presence of a king. Even more so is in a Shul where others may interpret it (although it maybe unconsciously) as an “OK” to use a cell phone in a Shul.
For all these reasons and more, it is only right to use a Siddur as Yidden have done for hundreds of years.