What is the cheapest way to use iphone (5, 5c, 5s, 6, 6s)?

I need a new smart phone and I'm interested in using iphone(5, 5c, 5s, 6, 6s). I'm not interested in iphone 4 because I heard they got no support anymore.

I seldom use call and web, say no more 20 minutes per month for calling, almost no chance to open web browser. (Basically I only use Wi-Fi.)

Would you mind advising me what is the cheapest plan for me?

The cheapest plan for you will probably be a pay-as-you-go plan. That's where you just pay a per-minute fee for calls and per-megabyte fee for data. It's best to just turn off mobile data and only use it in emergencies, because pay-per-use mobile data is extremely expensive.
It's hard to get any more specific than that, because you didn't specify what country you live in, and phone plans are different in every country. The only ones I might be able to mention are Vodafone and Virgin Mobile which exist in many countries, but they don't have good pay-as-you-go plans in every country that they operate. For example, Virgin has great pay-as-you-go rates in Canada, but they don't even offer it in the United States.

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