On the dial pad, you will see the main numerals, and to the rest, you will see a button with only symbols on it. Press the symbol key to access the + sign, then reverse the process to get back to the number pad. You might need to enter the IDD Code (+44, for example), then enter your number, omitting the 0 at the start of your number. If your number is, say, 07123 456789, then format it thus: (+44) 7123 456789.
