A call now button, or even a small icon, is definitely a good way to indicate to users that they can perform an action with the phone number.
It depends on your goals here:
If calling these numbers is central to your purpose/design, then you almost certainly want something that will call out the action that you can take with these numbers.
It is also possible that this is simply a listing of info about something, say a contact (or something more appropriate), where you are trying to give the user the number, not just to call, but for the sake of the information itself. In this case, it would make sense to have the number be clickable (indicated as a link, or some other designation). However, if you go this route, I would ensure that you or the system present a confirmation dialog to make sure that the user really wanted to call that number.
This is fairly standard behavior on smart devices - check out place/business listing applications that allow you to call a place/business as well as applications that help you manage contacts.
Also, if this is more centered around phone calls, such as in many phone apps (usually built into a phone), note that phone numbers there often take you directly to a phone call without any confirmation. This makes sense, as you are usually in a phone app to make a phone call, not because you were looking for some other information, such as an address, and found the phone number alongside it.
Since your application probably isn't a replacement for a phone app/dialer on a smart device, you would probably want a button, like your bosses say, to indicate to the user the action that they can take.