Some smartphones don't care for Outlook's phone number formatting and are unable to dialog calls when the number includes formatting or the country code.
Number formatting can be easily prevented in Outlook by typing the number in the field on the contact form and not using the Check Phone Number feature. If the country code is added as soon as you tab out of the number field, check the dialog properties. You may have it set to automatically update phones numbers with the country code. For more information, see Dialing phone numbers.
Smartphone users should avoid touching the Check Phone Number buttons as doing so will add a + and country code to the number. If you open the dialog, click Cancel to leave the screen, not Ok. Check Phone Number dialog
Before clicking the buttons beside the number, they are formatted without the country code:
When you click on the phone number buttons, this dialog opens.
When you click Ok to exit the Check Phone Number dialog, Outlook adds the country code:
You do not need to use the Check Phone Number Dialog to enter Extensions. You can enter extensions when you type the phone number.
To enter Extensions, type an x then the extension following the number:
How to Keep Outlook from Formatting Numbers
Additionally, if your phone doesn't handle the parentheses around the area code, switch to the International Freephone Service in Control Panel, Phone and Modem options.
Using this location, Outlook will leave your number as typed: 2021234567 provided you do not check the phone number, or if you do, click Cancel to close the Check Phone Number dialog.
This does not work in Outlook 2013. The only method I know of that will keep the format as one number, e.g. 4405551212, is entering too many numbers for your location; for example 44055512121 will stay as a single 11 digit number. The number can be dialed (unless your device blocks it) as the phone system ignores the extra number at the end.
Removing the Country Code from Existing contacts
If you don't have many contacts with the country code, editing each individual contact is workable. Unfortunately, there is no easy way to remove the country code from large numbers of existing contacts within Outlook.
You have two choices for converting numbers: VBA or Excel.
You can export the contacts to CSV format and remove the country code either by opening the CSV in Notepad or Excel and using search and replace, but if you have long notes, you may lose some of the notes.
If you prefer VBA, try the macro at Remove Prefix and Reformat Phone Numbers.