The lawyer, identified as Shamshul Haq, has been associated with the gang for the last 15 years, Senior Superintendent of Police (SSP) Vaibhav Krishna said.
He used to provide them legal-aid and help arrested gang members get bail, the officer said. The others arrested are Mithlesh, Alopi Mahto, Durga Mahto, Gopal and Nandan, all natives of Jharkhand, he said.
The gang used to steal the mobile phones and sell it in Jharkhand and Bihar, the officer said.
The gang members have confessed to selling around 700 phones in both the states, the SSP said.
Police have recovered 65 android phones and seized a car, a scooty and two knives from their possession.
The gang is wanted in several cases of theft in Delhi and Ghaziabad, the SSP said.