Earlier Airtel did offer 3G service even though they did not had license in Maharashtra using Idea/Vodafone 3G network. But it was not roaming for customer, as this network usage was happening at backend, while customer was displayed "Airtel 3G" on his handset.
Later after govt did not allow this arrangement, Airtel could not offer 3G plans in Maharashtra. But after TDSAT judgement, Airtel could be able to offer 3G plans again in Maharashtra.