Here is a list of old phones that will not be able to make phone calls when 3G shuts down soon, but will still have 4G internet data (and thus still useful to communicate via WhatsApp, Facebook Messenger, Telegram etc.). If you have one of these, please consider donating them to Hello Initiative, a social enterprise that supports young people involved in the WA justice system.
iPhone 5
iPhone 5C
iPhone 5S
Galaxy S5
Galaxy S6
Galaxy S6 Edge
Galaxy Note 4
Google Pixel 2 XL
Huawei Y6 Prime
Optus X Smart 4G
OPPO A57
OPPO F1s
If unsure, please refer to this article for more details.