Hello,
it is best to buy a hands-free attachment, usually used in cars.
Modern adapters usually communicate with the phone via Bluetooth, look for one that will work with your phone, read reviews, instructions, check in the store, etc.
The adapter has a loudspeaker output, you can connect it to the input of your sound system, perhaps through a simple divider to adjust the voltage levels, sometimes (although not very often) the adapters have a "line out" output, then directly to the amplifier, without any dividers.
Look for a snap-in with a "mute" output, which you can use to mute the radio, or switch to the input to which the snap-in will be connected.
It would not be difficult to look for a starter in which you can get the so-called "private mode" so that the audience does not hear your interlocutor through the speakers.
It will work as follows:
When you call or someone snap to you activates the "mute" signal, the radio switches, the speakers ring the phone.
You answer the call, switch to "private" mode and there is silence in the speakers.
best regards
Added after 4 [minutes]: leonov wrote: 
You need an amplifier, speakers and all that
I'm afraid you might be surprised.
Most phones have an inactive audio output until they are associated with an attachment: handsfree or headset.
