Some of the typical requirements for a multi-office LANs are:

• More balanced data transfers, including inter-site transfers;
• High speed data connectivity;
• A medium number of people requiring access at geographically diverse locations, typically with desktops and/or laptops;
• Access by smartphones/tablets may be required, but often restricted (site dependent);
• Telephone access between staff members without additional call charges, both intra-office and inter-office;
• Outgoing telephone connectivity from each staff to the general public;
• Incoming telephone access to dedicated staff (receptionist/s) or to IVRs, either centralised or site-dependent;

In this scenario, your network would comprise of routers/firewalls connecting to your ISP/s (often multiple at the main site and one router at each other site), an internal switched/routed network at each site connecting all the desktops/laptops, a dedicated medium/large VoIP PABX at the main site (potentially with smaller PABXs at the other sites), centralised email server and possibly other servers. Usually wireless connectivity is not required but may be provided on a site-by-site and limited in usage.

The firewall/routers will be a business-grade models (eg: An appropriate Cisco router) or servers configured as routers and firewalls and dedicated to that role (NOT used by any staff as a desktop PC). A DMZ would be established at the main site, where internet accessible servers would be located (such as email server, VoIP server, etc).