For old type flip phone, all basic functions(such as incoming call ringing, audio) should work.
However, advance features (such as voice assistance, dialing from home phone keypad..) may not work.
You can pickup the incoming call from your home phone but only can dial from your flip phone then pickup the home phone to talk.
Use separated power for your flip phone and keep it open (not in sleep mode) otherwise the Bluetooth will be off.
Suggest you should get a cheap smart phone (android or iPhone) which will provide you full features.