Open For Business
営業 means running a business. You're in the middle of running a business, which means you're open for business (otherwise you wouldn't be in the middle of running it, you'd be at the beginning or something).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will help to know 営業 first.
I asked you not to call me when the shop is open.