Internet dependency
Hosted 3CX
High — voice rides on the WAN. A failing primary link drops every extension unless a documented LTE or secondary fibre failover is in place.
Local PBX
Lower — internal calls stay up on the LAN even if the WAN is down. External calls still need a working SIP trunk or PSTN line.
Lean · Local where ISP quality is unreliable; hosted where you can stand up a second carrier.