Cox Cable Communications is in talks with Southwestern Bell to invest more than $ 30 million to join the regional telephone company in a venture that would buy and build cable systems in Britain, the Wall Street Journal reported.The discussions follow Southwestern Bell’s $ 60 million agreement this month to become the first regional phone company to buy U.S. cable systems. That plan may herald a new era of pitched competition — or cooperation — between the two rival industries. People familiar with the negotiations said Cox , the nation’s fifth largest cable companywith systems reaching 7 million homes, may invest $ 300 million or more for a stake in the United Kingdom venture. The two companies would then jointly build more systems in the nascent U.K. market. Executives at Southwestern Bell and Cox Cable, a unit of Cox Enterprises Inc. , declined to comment.