With 15 years of VoIP experience I’ve had a lot of learning curves.  First and foremost a successful VoIP deployment starts with a solid Local Area Network.  If you have old network switches, and older network cable, you are not ready for a VoIP deployment.  The physical network is the most important and expensive part of a successful VoIP installation.   I recommend HP\Aruba Switching if new switches are needed.  I am a big fan of the HP 2350 POE + series of switches.  After the physical network, we need to look at your Internet.  What is the stability of your current internet connection?  If your current connection is slow and unreliable then we stay away from SIP Trunks.  If your connection is stable then we look at what SIP Trunk provider works for you.  Last we look at what functions you need in your VoIP system today and 5 years from now.  My favorite VoIP system to date is 3cx.  It is extremely low cost, very feature rich, and runs on all platforms (Windows, Android, Linux, IOS).   The trick is finding an ROI by lowering your monthly reoccurring costs.  POTS (Plain old telephones lines) are expensive and non-feature rich, replacing them with SIP Trunks almost always pays for the system.  Also eliminating support calls to move a phone, or even change extension information can save over the years. Understanding that phone communication is one of the most need tools in business, we have perfected both failover and disaster recovery so you never miss an important call.