Can my Mac be a wireless speaker?
I don't know of any one OS X app that can do all of that for you, but here are a few recommendations. 

Turn your Mac into an AirPlay Speaker

Airfoil from Rouge Amoeba works great for me to stream podcasts, music, radio, etc. from iOS devices to my MacBook Pro (Mid 2012). Note that you'll need to also install Airfoil Speakers which is a free, separate download (scroll down to "Airfoil Speakers for Mac"). The separate Speakers download requires the purchase of Airfoil, even though the download is free - think of it as an add-on to Airfoil even though it runs as a separate app. Further note that AirPlay is a one-way transmission and cannot be used as a speakerphone. See the next section...

Turn your Mac into a Speakerphone

Not sure what kind of softphone you have but check with your IT or phone system admin to see if there's a Mac client available. If there is, you'll also need configuration details from them so ask for that too (or have them set it up). 

There are a couple of OS X apps that allow you to control your iPhone (and a few other types of phones) via Bluetooth but I've not had good luck with the reliability so I didn't purchase. I can't advise on those but a search online will turn up a few options if you'd like to try your luck. They've been around for awhile and are still under active development as far as I know. 

Good luck!