If your phone and your computer are both part of the Apple ecosystem, you can easily answer phone calls from your iPhone on your Mac by connecting your number to iCloud. Here's how to configure everything.
How to transfer cellular calls to your Mac
First of all, you have to make sure that all your devices are connected to iCloud on the same account where you can not share anything between them. You must also run iOS 8.1 or later on your phone and macOS Yosemite or later on your Mac. Both of your devices will need to be turned on and connected to the same WiFi network.
You must activate call forwarding on both devices. First, open the Settings app on your iPhone. Tap the "Phone" category, then tap "Calls on Other Devices." Make sure this option is enabled and your MacBook is also allowed to connect.
Then open the FaceTime application on Mac. Make sure you are connected to FaceTime and have your account activated. If you do, you can open the FaceTime preferences with Command + Comma or from the "FaceTime" drop-down menu in the top menu bar. Check the box labeled "Calls from the iPhone".
Your devices are now linked. If you want to add new devices in the future, you will need to allow them again in your iPhone settings.
Make and answer calls from your Mac
Answering a call is simple: Once your devices are connected, you are also notified of each phone call on your Mac. You can press accept or decline this notification to open FaceTime.
You can always pick up your phone, so your Mac will stop ringing and vice versa.
To make calls, you must open the FaceTime application. The list of recent calls and missed calls is displayed. You can click the phone icon next to one of the people on this list to call back.
If you need to make a new call, you must enter the name of their contact in the search field or their phone number (or Apple ID) directly, then press Call. When calling other FaceTime users, keep in mind that "FaceTime" is a video call and "FaceTime Audio" is a normal phone call.
How to disable call forwarding
If you find it annoying that all your devices ring simultaneously, you can turn it off from your iPhone settings because calls will still be routed through your iPhone. The process is the same as activating it; Open the Settings application, click "Phone," then "Calls on Other Devices," and then disable the "Allow calls on other devices" option.
If you want to prevent a specific device from ringing, you can also prevent that device from receiving calls in "Allow activated calls".
How to make calls without iPhone
If you do not have an iPhone, you can use a VoIP program such as Skype make cell phone calls from your Mac. You will need to pay your number but otherwise you will only be able to call other Apple users.