It means that when you are buying a phone, e.g: from a phone shop which is not a network like o2 or Tesco or orange. It will not have the network company's logo like 3 mobiles, they have the 3 logo behind. So no network logo means no logo of a network behind the mobile like 3!
The logo of the network usually is on the welcome screen, on the box that the phone came in with or you can check the settings of the phone.
Google uses it for their Android operating system.
What "are" sprint phones? Sprint is a phone company like T-Mobile or AT&T the phones that they use have the sprint logo on them.
That logo is Lacoste
Apple
WhatsApp messenger app has a green logo with telephone.
Logo with a green g
Maybach's logo includes the green and yellow M's.
Movistar's logo is a green M with subtle yellow highlights.
Iberdrola has a logo with a green leaf and two drops.
Green Giant