Dual phone lines, no redirect to second line.

My company has an issue with its multi line function for their phones. With the previous company, when phone line 1 was in use, calls would be redirected to our second line. Our problem currently is: when line 1 is busy, a new caller is sent to voicemail. I would like to have it set up, whereas line 1 is busy, new callers are directed to the second line. How do we go about fixing this issue?

Regards, 

Andrew