Cseries :: WiFi Sometimes Is Unable To Use?

I don't pay my service provider T-Mobile for internet acess on my phone, because I don't use Internet much and this phone already got Wi-Fi. So sometimes the phone's Wi-Fi worked just fine. But also sometimes it takes like 30 seconds (which only takes about 10 seconds when it worked) to load and then it said "Service not available". I had tried to use Wi-Fi with the phone closer to my modem with the Wi-Fi signal really strong, disconnected and reconnected, and turn the phone off and on again, but the problem sometimes happen, not always. So is this because of the phone's Wi-Fi receiver is bad, or something else?