It looks like Brave Girls are participating in a song for KHNP (Korea Hydro & Nuclear Power) sponsored Biz-Ring. In the video, it goes through the songs and artists that have been a part of it before and at the end reveals “Better Tomorrow” with Brave Girls. They appear at 21:37.
“BizRing” is a ringback tone service which offers a song or message instead of a connection tone. Customers of Korean phone services can apply to have the song be their ringback tone.