The cost saving that will be the main reason before licenses is the amount of hardware resources currently used, the system we are using in total is using around 128 GB of RAM, 64 cores of CPU, and total of 4TB of storage;
That is for just basic telephony for around 400 phones with call recording for some with a mostly non working HA and not to mention the DSP devices for PRI lines and media resources for phones which is using almost an entire rack.
If such can be reduced for two kamailio nodes and a couple freeswitch nodes for any IVR / conferencing / voicemail that would at least lower the resources to the half of what being used if not more, keeping in mind that Calls per second count
and concurrent calls count is really low.
To put this into perspective, to integrate the telephony system with MS Teams it will cost 70,000$ USD just for installation and basic configuration just to ring a deskphone when someone calls you on Teams..