(Photo by Justin Sullivan/Getty Images) Sprint customers are having a better experience on the T-Mobile network, but it's going to be quite a while until the two networks are truly merged, according to top T-Mobile executives who spoke at the Citi Global TMT West conference yesterday. "North of 20% of Sprint users are on the T-Mobile network," says T-Mobile President of Technology Neville Ray. He estimates that's a little more than 4 million customers, and describes "on the T-Mobile network" as Sprint users who spend "a majority of their time" on T-Mobile cell sites. But even the users who are still mostly on Sprint sites are seeing a better experience, as T-Mobile is filling in the gaps in Sprint's network with its own.