How to know if I'm texting another Verizon user?

I've been getting texts saying I have used about 90% of my MSG allowance. I pretty much text the two same people all the time and one of those people's number has a Verizon number. The other person's number also starts with a Verizon number and they recently got an IPhone and I remember him saying that he was going to let Verizon set up his IPhone, which means he uses Verizon. Does anyone know what the problem is?

Himini