Microsoft Teams is now a complete meeting and calling solution

Not sure if it's relevant to this post but in order for us to resolve S4B & Teams call feature issues for our clients, we worked extensively with MS for over a year to build a MS-certified Connector that addresses issues and missing legacy PBX features that businesses still need and MS does not see the need to develop. Legacy features such as analog, faxing, ACD Agent log-in/out, true enterprise extension dialing, external paging, etc. are still being used by many businesses.  We are developers and don't sell MS products and in order to observe non-solicitation rules, I can provide info regarding this Connector to community users who specifically ask me for it.

I also have a question and would like to know what challenges (if any) have anyone faced when attempting to utilize MS Teams as a full-functioning legacy PBX phone system or UCaaS replacement?  Why aren't more companies using MS Teams as a phone system since it is free and included in the E5/M5 license pack?  The alternative Cap-Ex cost of a premises-based IP phone system is at least $300-$500/user (100 user company: $30k-$50K just for the phone system alone) plus Telco lines and phone service costs.  With Teams Calling, if you have E5, phone system is included and you just need a MS or 3rd party calling plan for $10-$24/user.  Why isn't this being adopted more?