is call-forwarding illegal in prison, they blocked my number

I'm using a service called flymobi and I have there app that lets me take ćollect calls on it with a local phone number. It has to be online and when I can't be online, the app allows me to route my inmates local phone number to my cell phone, so when he calls, I never miss my calls. Does anyone know if this is legal? I got blocked from the jail because I was using call forwarding, but when I changed the number with flymobi, they told me not to use forwarding anymore and since then, everything's been fine. Not sure about this because it's strange that I went from paying $9 per call to 50 cents per call. I just like to know if this is legal.