Will the Voice Pro handle calls when the lines are busy?

There are two answers I need to give you, depending on if you are asking about in-house extensions or the incoming phone lines.

When an extension is busy, the caller will hear the voice mail greeting for that extension and then they can leave a message in that extension's mailbox, or then can go back to the main menu.

However, if all your incoming lines are in use, then the next caller will get a busy signal. There is no way to pass the call through to the VoicePro if you don't have enough phone lines to allow the overflow calls to get through.

If this is the case, you need to consider adding more lines and then use the larger VoicePro that handles four incoming lines.

If traffic volume is an issue and you don't want callers to get busy signals when more than four people call, then you need a bigger phone system. 

Of course this runs into bigger expenses, but if your traffic volume calls for it then hopefully you have the income from your business to warrant such a purchase. If not, another solution would be to consider a hosted PBX service where you pay only a small monthly service fee plus usage.