We are working on a list of community reuse projects, some of which accept mobiles, and can be trusted to wipe them.
I believe there are also credible corporates that wipe phones, but it requires some investigation. It depends on your personal risk threshold. Do you want data deleted immediately? (For example, weāre not certain that Mazuna wipes mobiles in the UK or merely does a triage here and exports them for data deletionā¦)
Restoring to factory settings will hide these from an opportunist but much or all of the information may still be recoverable with forensic tools, which criminals may well be in possession of.
We may not want to say this, as not sure this erm, effectively allows for reuse and recycling. Also, erm, lithium battery? 
@philip
Restoring to factory settings should then be effective, but even so, if youāre worried you might want to finish the job with a hammer!