To call someone on their work or mobile number, do this:
From the contacts screen, find the person in your Contacts list or type the person’s name in the search box.
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.