Using a phone during Tawaf is permissible when the phone is being used as a tool to aid the worship — such as reading Quran from a Quran application, accessing a dua collection, or using a counter app to track the circuits of Tawaf. These are modern tools that serve the same purpose as a printed dua book, which scholars have always permitted during Tawaf. Reading from a book during Tawaf was practiced by some of the Salaf.
Sheikh Ibn Uthaymeen, while he did not address smartphones specifically, stated that reading duas from a book during Tawaf is permissible. By extension, contemporary scholars permit using a phone application for the same purpose. The key is that the phone is being used for worship-related purposes, not for worldly distractions.
However, scholars unanimously discourage non-worship phone use during Tawaf: taking or making phone calls, sending messages, browsing social media, or taking selfies. These activities distract the pilgrim from the remembrance of Allah, which is the primary purpose of Tawaf. The Permanent Committee has advised pilgrims to minimize distractions during the rites and to focus their hearts and minds on worship. If a phone call is genuinely urgent, the pilgrim may step aside, as breaks during Tawaf are permissible, take the call, and then resume their Tawaf from where they stopped.