There are two versions for non-bluetooth phones with 2.5mm jack. One for a 3 conductor jack and one for a 4 conductor. Don't be fooled by the link for compatible phones, which advises you &quot;All phones with 2.5mm jack.&quot; Not true!! My phone is a Nokia 3500 series and it has a 4 conductor 2.5mm jack, and it doesn't work with this Jabra (model 100-91030000-02) I needed the model 100-91430000-02, and there's no distinction on Amazon's website. So this will be the 2nd return. First was my error in ordering the Jabra for a bluetooth phone. My phone is not.