To call someone on their work or mobile number, do this:
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From the contacts screen, find the person in your Contacts list or type the person’s name in the search box.
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When you find the contact, tap on their name, and then tap the Contact card icon (it’s the last icon). Then select the number you’d like to call─work or mobile number. This will call the person on their work line or mobile phone.
Depending on how you set your VoIP call options, when you make a call it could use Wi-Fi or your cellular data plan. For more information, go to Adjust your data use settings.