I will say if you put the NEC in place in the last couple of years (which I thought I saw that you did), it may be possible to sell it on eBay or to some kind of firm that buys old PBXs to make up a little bit of money toward new handsets / something else. When we got rid of our Avaya IP Office 406 system in 2013 and moved to Elastix, a company bought all the hardware from us for around $1500 - $2000 if memory serves (including about 70 Avaya 5410 desk phones as well).
Compare the cost of the system to the money saved over a period of 2-3 years switching to a SIP provider and using some type of open source PBX. You may make that money up pretty quickly.