WhatsApp

WhatsApp was a messaging service which Grace O'Brien used to connect with a group of other nurses. (TV: The Woman Who Fell to Earth) Its logo was a green speech bubble with a white outline and a white telephone icon in its centre. (WC: The Zygon Isolation)

Amira communicated with Baba using WhatsApp before losing her phone in the sea. (PROSE: The Stone House)

When the Thirteenth Doctor needed more information on strange events around Sheffield, Grace offered to check in with her nurses group on WhatsApp. (TV: The Woman Who Fell to Earth)

The Thirteenth Doctor later used WhatsApp to contact, who, according to C, had gone off the grid. He replied with a photograph of a fish, which the Doctor decrypted in the TARDIS using steganography. (TV: Spyfall)

Petronella Osgood had the application installed on her computer by 10 May 2020. (WC: The Zygon Isolation)

Behind the scenes
The Stone House incorrectly spells WhatsApp as "Whatsapp".