How do you safely dispose of old cellphones?

Wrace wrote:
My wife and I have a number of old cellphones that we no longer use and would like to donate or throw away. These range from old flip phones to a couple of 2010 smart phones. Some have battery chargers and some I doubt I could find the original charger if it even still exists here in the house somewhere.

We don’t mind donating the phones if some outfit can truly make use of them but our concern is how to make absolutely sure our personal data is erased from the phone permanently?

Thanks