Required Asterisk,FreePBX,VOIP Administration,VOIP Software freelancer for Which PBX Sip system? ( seeking purchase advice/ setup and administrtion) job

we have four separate network sites connected up via ssl vpn

all sites internet speed are minimum:
Download 50 mbps
Upload 15 mbps

Max Simultaneous calls at any one time: 20

Hardware we already own:
Vertical VW-E5000i-8 ip phones x 10 ip phones

existing infrastructure:
Old building with old wiring analogue phones qty 50 (not able to rewire the building)
20 office desk workstations that have cat 5 rj45 connections good for ip phones
of the above 20 about 10 employers work whilst out of office using android or apple mobiles

happy to answer any other questions

Skills & Expertise Required

Asterisk FreePBX VOIP Administration VOIP Software