To call someone on their work or mobile number, tap Contacts icon, second icon from the left.
Find the person in your Contacts list, or tap the Search icon at the top and type their name in the search box.
When you find the contact, tap their name, and then 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, see Manage Wi-Fi for VoIP options.