A musical instrument would be any instrument that produces music. So, what you really need is a good working definition of music. Now, there are many available on the internet and you are certainly welcome to search them till you find the one you like, or maybe there is some Islamic authority that already has THE answer for you. The one I will share with you is what I learned from my wife who was a music teacher.
Music is any rythmic arrangement of variable tones of different pitch produced by vibrating air or another medium to produce a particular sound. That's pretty broad. It isn't just the banging on a drum, because there is no modulation of pitch in a drum. And the sound of a police whistle isn't music either, even though it does change in pitch, because the variation in pitch determined by matter of chance of the bead in the whistle's chamber and is not a planned arrangement. But either could be used to produce music. The playing of more than one different drum of different picthes is definitely music. And a police whistle by itself might not be considered music, but it could be incorporated into a piece of music as one of several different instruments.
So, your Nokia ringtone. Does the pitch modulate? Is it arranged? If the answer is yes to both, and you have to manipulate it to get it to sound, then by the broadest definition used by those who teach music, it is a musical instrument, just like would also be a radio, or even a doorbell or car horn (though as you have already heard, it seems that someone in Islam has declared that the last two are not).
