Why does my high speed cellular data go fast only when I m on the phone?

Why does my high speed cellular data go fast only when I m on the phone? I noticed it today. It s the only way I can navigate Facebook and use apps that require data with ease. Its weird. I already called my cricket phone provider and we tried many steps to fix the problem but nothing works. My data connection is on, WiFi is off, roaming is off. I have 10gb of data too. I have a Nokia Lumia 1320 phone.

Read more: Why does my high speed cellular data go fast only when I m on the phone?