If you call someone on the iPhone without revealing your number, iOS 11 (and earlier) will give you a choice of options. For example, you have the option of permanently suppressing your telephone number for all calls or you can not even send your number with you on individual telephone calls. We'll tell you how to use both variants.
How to hide your phone number for everyone IPhone Calls
The easiest way to make sure your phone number is not sent is to turn off the feature from the Settings menu by going to Settings and tapping Phone, then select the entry "Send my caller ID." If you have received a mobile call, you will see a slider after a few seconds this to the left to suppress your phone number on all future phone calls. You can, however, reactivate the function at any time in the menu.
Alternatively, you can have your number suppressed directly by your mobile service provider. You can find information about this in the support area of your provider.
How to suppress your phone number for individual calls on the iPhone
If you do not want a permanent suppression of the phone number, you can not show your number with a simple trick, even on selected calls. To do this, open the phone app on your iPhone and select the "Number pad" tab. Before you enter the desired telephone number here, type in "# 31 #" before the area code. For example, the input may look like this: # 31 # 17612345678
Then tap the green handset to make your anonymous call.