Best pay as you go plan to get a new phone number

We got a tablet at the house and we'd like to add Whatsapp to it with its own number and are wondering what's the best/cheapest option for getting a new number and keeping it.

This number will never be actually used for anything other than verifying WhatsApp and whatever minimum activity I need to do to keep the number active. The tablet doesn't even have a sim card slot.

Is there any provider who has cheaper minimum top ups or longer times between requiring top ups? Or maybe 3 would be a decent shout cause I could use the credit to buy a game or something on the play store?

Thanks