The simplest, and most reliable way to add telephone numbers to your contacts is to ask your contact to telephone your mobile phone. After that call, whether you accept or decline it, the caller's number will be available to add to your contacts. Open the phone app and locate the favourites tab at bottom left of the screen. Swipe right once to locate the recents, tab and double tap to select it. Slide your finger near the top centre of the screen to locate one of two adjacent buttons - all and missed. If necessary, slide or swipe to locate the all, button and then double tap to select it. Now swipe right until you reach the first number in the list of calls and this should be your caller's number. If this isn't the number you want, continue swiping right until you reach the desired number. Swipe up once and you should hear more info. If not, keep swiping up until more info is spoken. Now double tap. This will open up a page showing information about the call. Now swipe right until you reach create new contact and double tap. This will take you to a text field to enter the first name. Type, or handwrite the first name. Dictation is also available, but may be unpredictable with names, especially last names. When you have finished entering the first name, type a return and this will take you to text field for entering the last name. Enter the last name and then slide your finger near top right of the screen to locate the Done, button. Double tap and this will create the new contact with that telephone number. If you already had this person in your contacts, perhaps with an email address, then there is also an option to add the telephone number to an existing contact.
If you aren't using VoiceOver, locate the call in your recent calls and the More Information button can be found to the right of the time or date of the call; it is a little circle enclosing the letter i. Tap on this to show the page which offers add to existing contact or create new contact.