Phone need to be able to receive/accept as many callers as the phone can take (on as many lines as it has)

Just imagine how our customers feel when they calling us and they constantly going to voicemail. Are they getting what they want ? Doubtfully. What are they thinking about our company? Lazy to pickup the phone or what ?-probably.
If your desk phone created with 10 lines for incoming calls to take - that's for a reason. Customers who calling us are in need of our service and when they do call most of them gets to the dead end.
Feature to be able to take as many calls as your phone is capable to - is a MUST.
Just ask yourself a question. If another provider can do it why can't you?
Or can you?