To call someone on their work or mobile number, tap Contacts, find the person in your Contacts list, or type the person’s name in the search box. When you find the contact, tap their name, and then tap either their 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.