The formula for induced voltage in a transformer is Vs = (Ns/Np)*Vp Vs is the voltage in the secondary coil, Vp the primary. Np and Ns are the number of turns of wire in the primary and secondary coils, respectively. The way a transformer works is an AC current is passed through the primary coil, creating constantly expanding and collapsing lines of flux. These lines of flux pass through the secondary coils inducing a current in them. The voltage produced as a result of the induced current is directly proportional to the ratio of the amount of turns of wire in the two coils. Not that it has nothing to do with the size of the wire, just the amount of pieces of copper passing through the flux. Often, transformers use the smallest wire possible that will still support the amount of current needed.
A cell phone with a 1GHz processor has hundreds of thousands, if not millions or more tiny copper conductors. If a line of flux crosses all of them, then they will each induce a potentially damaging current, especially considering that an increase of 1-2V will create sufficient current to severely damage most handheld electronics.
A power transmission line is effectively one conductor (it may be a stranded conductor, but that's considered to be connected in parallel, thus the voltage is equal in all conductors, not added together) and is generally in the 3-25kVAC range before the transformer and 120-240VAC after the transformer (these are all RMS values of course, hook an oscilloscope up to a 120VAC power source and you'll see closer to 170Vp, which is 340V from absolute maximum to absolute minimum).
So let's do some math. Let's say an EMP drives a pulse of 1Wb/s, which when passing through a conductor will induce a voltage of 1V. So now the processor in the mobile device, which is used to 1V, is now randomly dealing with voltages anywhere from -1 to 2V. This can either exceed the maximum forward or reverse voltage in the transistors, thus destroying the processor. The change of 1V in the 3.3kVAC transmission line is not even noticeable in the background noise.
Increase that pulse to 100Wb/s, now you're creating 100V in each conductor (note, I'm not considering losses due to distance, let's assume you're the strength is measured at the location not the source). The mobile device, due to the density of the wiring, may start arcing between traces, maybe even give whoever is holding it a bit of a shock. The extra 100V may be enough to pop your household breaker, maybe a couple fuses in your more sensitive electronics but I bet your stove and fridge will still work. The 3.3kV transmission line may see a noticeable blip, but nothing out of the ordinary still.
You would need a pulse in the range of thousands of webers/second to do serious damage to power transmission systems and large household appliances, and by then, your iPhone would become a pretty crispy critter too.