Virgin Mobile answered your question in the FAQs:
• About Removable UICC Cards
All 4G LTE (non WiMax) phones require a Universal Integrated Circuit Card (UICC).
• What is a UICC card?
A UICC card is the physical smart card that is pre-inserted on your device and allows for LTE network authentication/ access. It's a new generation SIM (Subscriber Identification Module) card. No customer-specific information-like contacts, address book, or account information, photos, or personal data- is stored on this card.
Virgin Mobile USA phones without 4G LTE don't use SIM cards.
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