The cheapest base rate plan is what it is. What are you looking for if you don't want to pay 15? You do have the option of not paying until the 89th day and then topping up again. Then it's closer to 5 bucks a month but you can't make calls without an active base plan. If you just want a number then a VoIP provider may be better for you

Or look at 7-11 Speakout. You can activate a line for only $25/year, but of course you don’t get any extra minutes or data. Good for an emergency line though.