If you cannot get the unofficial WhatsApp client working, or if you prefer more stable messaging solutions: The most reliable method on any feature phone.
If you are using a Nokia 105 or similar phone, remember what these devices were made for: . Modern SMS plans are incredibly cheap. You won’t have read receipts or groups, but you’ll have reliability and no battery drain. Whatsapp java j2me
WhatsApp for Java J2ME: The History of Mobile Messaging on Legacy Devices If you cannot get the unofficial WhatsApp client