How Many Lines
You should ask yourself one question when deciding how many lines you need.
IS MY COMPANY'S MAIN TELEPHONE NUMBER EVER ENGAGED?
If your answer is anything other than NEVER consider connecting more lines.
The cost of renting telephone lines (or ISDN Channels), compared to the cost of generating new business customers to ring you, is insignificant.
Good customer service (keeping those hard earned customers) is equally as important.
The mobile phone companies, Orange, T-mobile etc., are perfect examples of how not to do it.