Will whatsapp use my credit on a pay as you go sim?

WhatsApp itself does not charge you for sending messages or making calls, so using the app won't directly use your credit on a pay-as-you-go SIM. However, WhatsApp requires an internet connection, so you will use your mobile data when sending messages or making calls over the app. If you have a limited data plan, that data usage will count against your allowance. Always check your data usage to avoid unexpected charges.