Welcome @Mehlk
WhatsApp is a popular choice . It’s easy to use ( no passwords or logins required ) . You simply exchange mobile phone numbers and then you can have a video call .
Then , if the sit is confirmed , WhatsApp messages are a good way to keep in touch and for a sitter to share photos and updates during the sit .
WhatsApp does not require either party to have mobile data , (just an internet connection ) which is useful for international sits .