How to request DNS flush of MCI/Verizon DNS Server?

[Webdeveloper]

I work for a website company and have a customer that is using MCI/Verizon Business internet.

48 hours ago we change the name servers for the domain to point to new nameservers, but the DNS Server their internal network is using is still pointing to the old IP addresss.

The DNS servers reported by a computer on their network are
198.6.100.6
198.6.1.125

which are owned by MCI/Verizon

The domain in question is nissanofbowie.com

Your name servers are returning

C:\Users\me>nslookup
Default Server: dns.google
Address: 2001:4860:4860::8888

> server 198.6.100.6
Default Server: dialcache061.ns.uu.net
Address: 198.6.100.6

> nissanofbowie.com
Server: dialcache061.ns.uu.net
Address: 198.6.100.6

Non-authoritative answer:
Name: nissanofbowie.com
Addresses: 13.225.206.25
13.225.206.90
13.225.206.101
13.225.206.81

vs public name servers like Google/Cloudflare returning

C:\Users\me>nslookup
Default Server: dns.google
Address: 2001:4860:4860::8888

> nissanofbowie.com
Server: dns.google
Address: 2001:4860:4860::8888

Non-authoritative answer:
Name: nissanofbowie.com
Addresses: 2606:4700::6811:2596
2606:4700::6811:2896
2606:4700::6811:2996
2606:4700::6811:2696
2606:4700::6811:2796
104.17.40.150
104.17.39.150
104.17.41.150
104.17.38.150
104.17.37.150

[Cang_Household]

The DNS servers in NY Metropolitan area are returning the same addresses as Google DNS.

DNS propagation may take as long as 72 hours. I would give it another 24 hours of waiting.

Is your customer on copper of fiber?