I’m dating myself a bit, but I use to install T1s and cisco IP phone systems for Telesphere before they were bought out by vonage for business... Large (I’m talking 500-1500 phones with multiple T1 or larger feeds) offices used them. VOIP is much cheaper than POTS lines.
We worked on similar projects, the largest site I worked on was 8000 phones, over VG 224s to handle analog devices, the site was a 20 building campus, we didn’t convert it in one weekend, we had 2-3 conversions a month of small chunks,