The first method will guide you to add an extension to one contact’s phone number in the List, Phone, Business Card, or Card view in Outlook.

1. Open the contact folder containing the specified contact, and change folder view by clicking View > Change View > List, Phone, Business Card, or Card. See screenshot:

2. Double click to open the specified contact whose phone number you will add extension to.

3. In the contact window, please (1) click the Business field button (or other filed button as you need) in the Phone numbers section, (2) in the Check Phone Number dialog box type the extension into the Extension box, and (3) click the OK button. See screenshot:

4. Now the extension is added to the specified phone number, please click Contact > Save & Close to save changes and close window.

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This method will show you the way about adding an extension to the phone number of an Outlook contact in People view. Please do as follows:

1. Open the contact folder containing the specified contact, and change folder view by clicking View > Change View > People.

2. Click the specified contact to preview it in Reading Pane, and then click Edit in the header section. See screenshot:

3. Now the contact information is editable. Please go to the specified phone number box you will add an extension, and add space + x + space + extension number behind original phone number, such as  x 153 as below screenshot shown.

4. Click the Save button at the bottom of reading pane to save the change.

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