If you want to activate the speakerphone, you should be able to do so while you are in the middle of a call. You can see all the different options like this available the next time you are on a call.

More info on this can be found in the user manual, under In-call options. You can find the link to download that here on this website. The user manual here on the A157 page (scroll near the bottom of the page).

Let me know if that helped you?