Can I withhold number with an iPhone 5s? I need to join a client's conference call today but I'm likely to be out of the office. I don't have a company phone but don't want to share my personal mobile number. Is there a way I can withhold my number so they don't get my personal mobile number?

I've been looking in the Settings/Cellular and Settings/Network but can't see anything promising.
Look for Settings → Phone → Show My Called ID. Depending on your carrier this may or may not be available.

If it's not available there might be another way to hide your caller ID by pre-pending a prefix to the number you dial. For example in the UK you can add 141 to the start to hide your called ID.