I presume you mean a 3.5mm to 3.5mm stereo jack plug lead, which any half decent audio, TV or hi-fi shop should have in stock in various lengths at low cost - you could even try a discount store that offers audio accessories.
I think you need more than just a cable. In fact I think what you want to achieve may not even be possible.
For a start, the output of the computer is nothing like a microphone signal - it's much louder for a start. Also you'd need to know how the 'phone is supposed to use it's jackplug input (if it is an input).
But why do it through a phone - loudspeakers or a decent headset would be guaranteed to connect and give you the right signal levels and a good sound (better than a phone ever will.)
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