Three US Senate Democrats have asked the four major wireless carriers about allegations they've been throttling video services and—in the case of Sprint—the senators asked about alleged throttling of Skype video calls.

Sens. Edward Markey (D-Mass.), Richard Blumenthal (D-Conn.), and Ron Wyden (D-Ore.) sent the letters to AT&T, Verizon, Sprint, and T-Mobile, noting that recent research using the Wehe testing platform found indications of throttling by all four carriers.

"All online traffic should be treated equally, and Internet service providers should not discriminate against particular content or applications for competitive advantage purposes or otherwise," the senators wrote.

Specifically, the Wehe tests "indicated throttling on AT&T for YouTube, Netflix, and NBC Sports... throttling on Verizon for Amazon Prime, YouTube, and Netflix... throttling on Sprint for YouTube, Netflix, Amazon Prime, and Skype Video calls... [and] delayed throttling, or boosting, on T-Mobile for Netflix, NBC Sports, and Amazon Prime by providing un-throttled streaming at the beginning of the connection, and then subsequently throttling the connection," the senators' letters said.