To hear voice calls from Bluetooth Speaker I am able to hear MP3 songs from my Android phone using a Philips BT50 Bluetooth Speaker. However, when I make and receive calls from my Android phone the output does not comes from the Bluetooth speaker; it just directly comes from the phone's speakers.

Is there a way to hear voice calls from a Bluetooth speaker?
Yes, you need a Bluetooth router. There are example apps in the play store.
BTMono, or BAR are example options.