"N900 refuses to vibrate - I'm getting desparate"
One could take that title -entirely- wrongly. XD

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I'm guessing the reason the OP said motor is that a lot of phone vibrators(all of the ones I've seen) use a micro brushed DC motor and a "half-sphere" weight on the shaft. As it spins, centrifugal force makes the whole thing shake, as its not even.

I could see using a solenoid for this purpose also, but it would require an electronic control circuit etc. to make it work. Not just a single FET for turning it off and on.