Do you mean you reduce voltage with potentiometer?It wouldn't work, as phone drain a lot of current, especially onstart up. Add up transistor, better darlington, or drop voltageinserting two diods in series : 5 - ( 0.7 + 0.7 ) = 3.6 V
It is not clear what role the Arduino has in any of this? It seems rather risky to even have it in the circuit, IMHO.It would appear that your circuit to produce 3.6-3.7V does not work. You could confirm that with your meter.If you would like assistance with your voltage regulating circuit, you can start by showing us the schematic diagram.