BT had sued Google back in 2011 on 6 patents related to mobile technology, claiming that Google infringes on the patents in Android, Gmail, and Google Maps. Google says that not only are the claims baseless, but that BT has been arming patent trolls as well. Google has also been sued by Suffolk Technologies LLC and one of the patents being asserted by Suffolk was previously owned by BT.
Google's lawsuit asserts that BT has infringed on 4 of Google's patents that cover technology such as networking and making phone calls over the Internet. At least one of the patents in Google's lawsuit was purchased from IBM back in 2010. Google has also apparently filed a suit against BT in a UK court.