A record share of U.S. corporate-bond trading has moved to computers as buyers who traditionally transacted over the phone seek faster ways to buy and sell in a market where Wall Street's human traders are retreating.

Investment-grade volumes on MarketAxess Holdings Inc.'s electronic system are on pace to exceed $400 billion in 2013 after surging 45 percent to $44 billion in September from a year earlier, according to data from the company, which estimates it captures about 90 percent of electronic trades among the dollar- denominated notes. That's equal to 14.3 percent of all market activity, including business done over the phone, up from 12.2 percent a year earlier.

Investors are seeking alternatives to establishing bond prices by calling multiple Wall Street firms, which experienced an estimated 20 percent decline in debt-trading revenue in the third quarter as they curb holdings used to facilitate trades. U.S. corporate-bond buyers expect to conduct 30 percent of their volume electronically by 2015, according to McKinsey & Co. and Greenwich Associates.