I had similar issue, but I found out what needs to be done, at least what worked out for me.
If you have iCloud account and apple id that are different from each other, meaning different email addresses, you will need to make sure that when you sign on to FaceTime on your Mac to use the same apple id that you are using in the iCloud settings on your phone. If you go to settings on your phone and scroll down until you see iCloud you will notice an email address under that, which is the cloud id used on your phone. Go to your Mac and click on FaceTime, once opened, click on the preferences on the drop down menu, and you will see the apple id used to sign on to the face time. Assuming you are already signed on. If it is a different apple id, then sign out and sign back in with the same one on your phone cloud. It will ask you to select your phone number and any other emails to use for phone calls. Once you click "next" it will sign you in.
Second, after you are signed in, click preferences from the drop down menu and make sure that the "iPhone cellular calls" is selected.
To make outgoing phone calls to anyone from your Mac, including land lines, you can use your contacts app or the FaceTime app on your Mac. from the FaceTime app use click on the audio tab to the right above, and then type in the search bar the name of the person in your contact list. You will find a phone symbol next to the name, click on it, not the video symbol, and then pop up small screen will show with different numbers of that contact, if applicable, or just one number. It will say on top of that "FaceTime Audio" or "Call Using iPhone" (depending on whether you stored the contact number as an iPhone or as a mobile or something else). If you click on any of the numbers then you will be using your phone network to make the call, like in a regular phone call, but if you click on FaceTime Audio itself, then you will be using wifi and no phone network minutes will be accounted in that phone call. This is useful to call someone overseas for free. I tried it and it works. Similar to FaceTime with a video except that sometimes the video quality would not work well for overseas calls as broadband might not be available. BTW, this is also extremely helpful and useful for anyone traveling on an airplane and wants to call someone using FaceTime. Basically, you don't have to have cellular network, only wifi through the airplane, and you will be able to call someone using FaceTime audio, or video, from your laptop or your iPhone. Of course the receiving party must have FaceTime with wifi, 4G or LTE.
From your contact app, just go the the contact you want to call, and then hover with the mouse over the phone number and you will see a phone symbol appearing to the right. Slide your mouse over it and click it and you will be making a phone call. FaceTime audio and FaceTime video are clear on the contacts, just click the one you want to use if they have iPhones and you want to reach with that.
