This won't work well because the microphone-level input for your cell phone is MUCH higher than the audio level your scanner puts out.
Have you considered setting up a streaming audio server? Products like Shoutcast and Windows Media Encoder are free and let you set up a much higher quality stream than a telephone; you can plug your scanner's headphone output in to your computer's line in jack.
If you really want to use the phone, you can patch your cable through something like this to lower the audio level down to something that won't overdrive your cell phone's mic input:
Gold-Plated Volume Control Cable for Stereo Headphones - RadioShack.com