The company had considered various schemes to offset its carbon output, but had rejected them all in favour of cutting emissions directly.
Vodafone expects to make the cuts by more energy-efficient manufacturing and increased use of renewable energy.
Chief executive Arun Sarin said: "This is the right decision for a responsible business to take.
"It is good for the environment but also makes sense for the business. We hope that making this public commitment now will encourage others to take similar action."
Vodafone estimates its carbon output at 1.23 million tonnes a year, around four-fifths from powering up its phone network.
Jonathon Porritt, founder director of Forum for the Future, which works with Vodafone on sustainability issues, said: "This is a significant commitment from one of the world's largest companies.
"2020 may sound a long way away, but it is what Vodafone does over the next two or three years which will determine whether or not it hits that 50 per cent target."
Vodafone pledges emissions cut
By Iain Thomson on Apr 25, 2008 12:58PM