I would show a "Skip" button.
- This reinforces that the step is optional.
- It aligns more readily with what the user is likely thinking.
It might be worth being even more specific by labeling the button "Skip this step" or, even better, "I'd rather not share my mobile number." (Or, for you Melville fans: "I would prefer not to.")
It would also be good, while you're at it, to say why you want the mobile number at all, so users can make an informed decision while confident you're not just asking them for all the information you can grab, and what will happen if they opt out.
So the final experience might look like this:
download bmml source – Wireframes created with Balsamiq Mockups