WhatsApp was a messaging service which Grace O'Brien used to connect with a group of other nurses.
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 "O", 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)
Behind the scenes
The Stone House incorrectly spells WhatsApp as "Whatsapp".