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.
I think they're just adding the minimum amount to prevent the minutes expiring and the phone subsequently being cancelled, which I assume happens after 3 months. They would still have to typical rates pay per minute or MB for any usage.
I add the minimum. In many countries (Switzerland, UK, New Zealand) the minutes don't expire, and the number is mine to keep unless it doesn't connect to a network at all for a year or more.
Yes, I pay per minute for outgoing calls, or MB of usage - and those costs are high. That's why I try to use WiFi whenever possible, and use offline maps, offline Wikipedia, etc.
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.