Why isn't iMessage Activating?

My sister got a new iphone and gave me her old CDMA iphone 4, without a current cellular carrier (phone linked to verizon), with idea I could also use it as an ipod so we could send imessages via wi-fi. I was able to connect to my wi-fi, set up an icloud account, download some apps, send e-mail messages, access internet - but when I tried to set up imessages, I get the "waiting for activation" message. Read all blogs I could find on CDMA iphones and on activation problem, but found no fix or explanation for this particular problem.