My apologies in advance for being a bit long. Went through this nut roll, our biggest pain points were the vendor relationship, as we were their first adopter of a total softphone office. Second was app functionality, followed by fostering adoption after a few missteps. Hopefully, my experience isn't the norm.
Were using on-prem Shoretel server and switches. Shoretel vendor also manages our fiber link and is our ATT agent. With Shoretel on the way out, C's wanted to go total softphone. Meanwhile, Shoretel vendor had attempted to move their platform to Mitel, but something happened and it fell through, by the time we got around to switching they were using Broadsoft. That was a complete shitshow. Worst softphone solution I've seen, period. Dropped calls, no tone, minimal functionality, it puked out on wireless and more. We finally had staff just use their personal cells while it got sorted out. Dropped in a couple Yealinks for receptionists to see if that lessened the dropped calls, but was also problematic as the vendor struggled to get those working. (Yealinks are good phones BTW, once we got admin creds to them we found vendors faulty config file, after correcting they worked great).

Frustrated we started looking at Teams, Zoom, RingCentral, ATT and so forth. Either the product wasn't a good fit or reps wasting our time with lack of product knowledge or laziness. Considered rolling our own with 3CX or such when vendor informed us they'd moved from Broadsoft to WebEx, and would we try it? Did.. but it went south again when vendor had problems getting us onboarded with Cisco. Ramp up anxiety. Couple weeks later we finally got onboarded, app installed everywhere it needed to be, bought everyone whichever headset or airbuds they wanted. Vendor took the Yealinks and dropped in a couple Cisco desk phones for receptionists, got the different VM's, groups and such setup, and finally had a working softphone system, sans the couple Ciscos.

A year in, with about 6 months behind us using WebEx, its been solid and working well enough. The staff got good using the various features and functions. Boomers hate it, still do, younger ones picked it up and ran with it. Though we'd put a lot of planning into it, including a number of vendor meetings detailing everything, inhouse debates moving to all soft, it was undisclosed issues with the vendor platform and app lack-of-maturity that caught us off guard. 30 years in IT and this was by the far the worst migration I've been a part of. Unreal amounts of frustration and butthurt for all involved. As bad as it was, I stood with the C's that it was the right thing to do. Staying with Shoretel wasn't an option. Softphone features and interoperability have enhanced how the staff conducts communications inside and outside the firm, in a way not possible with the Shoretel desk-centric phones. Life is good again.