While the mobile phone number is a field that can be imported it is not a field that has a built in contact details name. This seems like it may have been an oversight to me and I've brought this to the attention of our developers.
I did verify that there is a Subscriber.WorkPhoneNumber field that can be used, or you can create a custom field which you might use a name such as "mobile" and that would look like Subscriber.Custom.Mobile. When creating a custom field do not use the exact name of a field that Constant Contact uses already, so do not name your field "mobile phone" or "mobile phone number". You can find info on how to create a custom field here. https://knowledgebase.constantcontact.com/articles/KnowledgeBase/5328-add-and-manage-custom-fields