Cable companies have begun offering wireless phone service that uses networks built by big mobile carriers — a trend that could ease the way for Bellevue-based T-Mobile US and Sprint to merge.

That’s because regulators are more likely to approve the deal if they conclude there will still be sufficient competition even if the third- and fourth-biggest mobile providers combine.​

They are examining the idea. One of the regulators deciding whether to allow the merger, the Federal Communications Commission, this week asked the largest cable companies — Comcast and Charter Communications — for details about their new mobile offerings.

“If anything this is a positive sign for the T-Mobile-Sprint deal,” Amy Yong, an analyst with Macquarie, said in an interview. “It’s an option the FCC can point to as it serves its role in protecting consumers.”