? Plugged into the headset socket. Test using the headset key.
? Broken handset. Test with one from another phone.
? Broken phone. Try the handset on another phone.
? Something in the handset space so it is not actually getting close enough to activate the hookswitch (apart from the usual office fluff, the J100s use magnetic hookswitches so that handsets have been known to collect small items like paper clips and staples).
If you've checked any of those it would have been useful to have told us, also to have said what happens (if anything) when lifting/replacing the handset from the phone.
Stuck in a never ending cycle of file copying.