I keep a few old ones (even those for carriers I don't use or that can't be used (I have a couple of MicroTACs and the best phone ever made, the StarTAC, and a few other old phones. But the only one that can be used is an old Motorola V551 that I keep as a spare. All the other usable phones I gave to womens' shelters. (I used to own a few cellphone stores and took trade-ins. If the trade-in wasn't worth much, it went into the donate box, and a different shelter got the phones - with chargers and good batteries - every month.)
If I ever replace my Note 3 (I doubt I will, but who knows), the V551 will go to a shelter. (Let's face it - we will unfortunately never run out of battered women, so even a little help is better than none, and I really don't need the $10 I can get for the phone.)
Which reminds me - I have an old LG I have to find the charger for and bring to a shelter.