A SIM swap is a process by which mobile network operators transfer a phone number from one SIM card to another. This is prompted by several factors, including losing the original card, technological upgrades, or other circumstances. The implications of SIM swaps for the security of phone numbers are significant, as they expose owners to risks such as identity theft and financial loss. We characterize a generic SIM swap process as a security ceremony to address such challenges imposed by human-human interactions on top of security protocols. We model the resulting ceremony using the Tamarin prover to concretely identify authentication challenges arising from the introduction of malicious impersonators. We propose a social-based authentication scheme for the SIM swap ceremony to mitigate such challenges and similar attacks described in the literature. Finally, we formally show that our newly proposed ceremony can resist the current attacks.