Connextions — a subsidiary of UnitedHealth Group Inc.’s Optum health services business — plans to open a large facility in Irving that could employ hundreds of workers. “It’s not a call center, but it’s something similar,” said Kim Whitaker, a spokeswoman for Minnesota-based UnitedHealth on Thursday evening. “It will be more than that.” The company has leased Regent II Office Center, a two-story, 76,400-square-foot facility at 5150 Regent Blvd. in Irving, according to real estate…