I use an app called "Burner"
[QUOTE=Smokarz_XO;2547709]Get the Google Voice app. I'be been using it for years, and love it. No need for a burner phone.[/QUOTE]Not free, but lets you send pics back and forth, have different numbers for different shit and has a passcode if you want.
Second phone pros and cons
Personally, I prefer to keep a second phone. It keeps everything separate, and when the warden asks to borrow my phone I don't have to worry about what she might find.
I use T-mobile and pay $50/ mo for unlimited text / voice / data, and I didn't provide ANY personal information when starting. The cost is well worth the additional safety and freedom it gives me.
You do have the issue of what to say if the warden ever finds the second phone. I already have a very plausible cover story for that if it ever happens. You don't want the SO to think you're Heisenberg or something!
I also use Google voice that is forwarded to my burner. This way if a girl becomes a stalker (I had one many years ago), I can block her number easily. Only a few girls have ever had my direct burner number.