Shhhh. I'm trying to set up an anonymous phone number, and this is a very delicate process.
Oh, what's that? It's not? Hushed? You hush! Oh... I should check out Hushed.
According to this Android Police post, Hushed lets you create a disposable phone number without dealing with monthly fees. You just pay an upfront amount to use a line for a preset time period starting at $2 for 7 days. Each pricing tier comes with a limited number of minutes and messages that you can use before paying again. The lifetime plan includes a combination of 500 minutes and 1100 messages per year, just like the usual 365 day plan.
The deal expires in four days. Hmm, better hurry... quietly.