Prepaid where the credit expires after 21 days sounds more like a subscription that lasts 3 weeks and cancels safely.
With my current prepaid SIM, I typically top up $5 every 3 months. Between iMessage, WhatsApp/WeChat/KakaoTalk/Skype/Facebook, free incoming calls, and the occasional emergency SMS, I usually don't need a phone. People don't like it when I don't have a local number though.