When you calculated those savings was that a straight up comparison of your UCaaS MRC vs your SIP Trunking MRC? Because if that is the case, that is skewed. You now have a software PBX that requires a server, power, space, etc. etc. all of those things are a factor. In some cases they could be a bigger factor than others. You also have the cost of being the admin of the PBX. So anything that your provider may have done at your request, you now have to do. So your time is now a factor in this calculation. Updates, changes, support for issues that happen.
Now that said, UCaaS could cost more than having a onprem PBX system but there are various factors that need to be considered. I offer both services in the markets I’m in and they both have their advantages and disadvantages. In some cases UCaaS works out better for them cost wise than their onprem PBX. However I should clarify I’m in markets where telecom level support is expected and desired (24/7 support, onsite calls/truck rolls, etc). So 95% of the onprem PBX systems are hardware based for those I supply SIP Trunks to whether is has a PRI or SIP interface on it.
Something else to consider with onprem, from my perspective, I have less than a dozen places where I am End-to-End (Voice/onprem PBX). Roughly a 1/4 of them are FreePBX and the rest are hardware based systems. There is more cost to the maintenance of those FreePBX systems than there is for those hardware systems. By that I mean I don’t have to touch those other systems at all once they are up unless it’s a programming change (which is rare). On the flip side, I’m in those FreePBX boxes at least once a month doing updates and maintaining them.
Again, MRC to MRC from the carrier there is a difference but you always have to calculate everything else that you take on when doing onprem systems.
And I haven’t even touched on the costs of backups or any DR plans/policies.