There can be so many things that you have to remember while designing your business cards that it might be easy for something or the other to slip your mind. This can be disastrous since it can result in your entire batch of cards being worthless at this current point in time, but by being proactive and understanding all of the requirements by listing them down you can reduce the likelihood that you would forget about them and you would instead be able to get them sorted out well beforehand without a shadow of a doubt.
There is one thing in particular that people forget, and that is the basis for why Metal Business Cards exist in the first place. They are meant to be purveyors of information, and as a result of the fact that this is the case the information you add to the cards should take precedence over other things such as what they look like from an aesthetic point of view. You might get so obsessed with beautifying your business cards that you would forget the main purpose that they are supposed to serve, and it is essential that you stop yourself from being so negligent.
You would ideally want people to start getting in touch with you and inquiring about your services after they receive a card from you, and that is why adding a phone number can be so useful from every single angle that you can think of. Customers will be more likely to hire you for something or the other if they have a phone number they can dial in, and that makes this an essential component of your cards.