Vodafone has been given a 30-day deadline to pay a $2.5bn tax bill the Indian authorities say it owes following the acquisition of Hutchison Whampoa assets three years ago.The mobile phone giant is appealing against the decision, claiming the $11bn deal is not subject to taxation as it was struck between two offshore entities.British firm Vodafone bought the Indian telecoms assets of Hong Kong conglomerate Hutchison Whampoa in 2007."Vodafone strongly disagrees with the tax calculation," the company said in a statement after receiving its first formal tax demand from the Indian tax department."The tax authority is attempting to interpret Indian law as it has never been interpreted for the past 50 years, and this interpretation also goes against internationally recognised tax norms."Vodafone's boss Vittorio Colao is not a happy man. He's paying $2.6bn for a 3G licence and investing another $500m in the network."We need to get more certainty that regulation will not come back and bite us in order to confirm our investment," said he told India's Economic Times newspaper."I have actually invested more in India because I do believe in the country, but of course now I also need a positive outcome from the tax case and stable regulatory environment to continue."