Iphone 4 (Verizon) version 6.1.3 - FaceTime not working

Hi, I have an issue with facetime not working on my iphone 4. It worked fine until a few weeks ago. I have done a "soft restart" (power & home button) on the iphone. I have signed out of facetime and imesage, and then signed back in. I have checked the settings, and everything looks like it's right. The phone number has a checkmark next to it, but is greyed out under "You can be reached by FaceTime at:", but I read that it's normal. The imessage works fine. I do have an ipad air on the same apple ID, could that be it? but it was fine before. I just got the iphone 4 replaced at the Apple store via warranty, because the front facing camera was acting up. What else can I check? What else can be wrong? I do not want to update to the IOS 7.1, because the Iphone 4 will lag in IOS 7.1 update.