Add 2 mobile numbers to iphone
A custom label will have to suffice here. I have many contacts with their cell phones listed as 'mobile', and then a custom 'Google Voice' labeled number.

[edit]
An alternative to this is to make the change via Address Book.app on your computer, then sync the contacts to your phone.

Custom labels are available in all contact adding/modification screens.

First, browse to a contact, and tap the label name (to the left of a phone number, e-mail address, etc.). You will see the list of labels.



Second, swipe all the way to the bottom, and tap on "Add Custom Label".



Type in a new Custom Label;



And then it will be displayed next to the number.



It will now appear in a list of custom labels on later access.