Cisco, Avaya and Mitel are the "obvious" choices for a certain class of customer that still thinks they want or need a traditional, on premise system and a big name.
There are at least a dozen Asterisk flavors to choose from and work with to create your own on premise or cloud or hybrid solutions.
Zultys is a great choice for a similar experience to NEC from the reseller perspective as well as having solid on premise, VM and hosted options.
Grandstream maybe for small and cheap? They have unbelievably cheap hardware and just announced a cloud platform last week and seem reasonably capable. I've been evaluating their new network stack lately and it is capable, but so inexpensive I still can't really believe it. WiFi 6E APs <$100, 8 portPOE switch <$30, Gig router with POE ports and 2x SFP for <$80. This all has a free, optional cloud management portal and mobile app for management and monitoring too.
Also stay in touch with your NEC reps and see what they are going to do to help bridge the transition and dig into Universe Blue if you want to stay NEC. My understanding is they may be the only path to allow you to reuse NEC desk phones easily.
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