Is this project possible, especially for two or three clients on the same server -- this server can be located in a virtual machine? Can you suggest how I could start this project, as I am a beginner in VoIP?
The network design piece requires more thought. You have to understand how many locations you will have. You need to ensure that in each major office you have a tiered switching network -- edge switches, maybe with Power over Ethernet (PoE) and UPS systems for backup -- to power the IP phones, mid layer switches and backbone switches. You typically only need Quality of Service (QoS) on the wide area network (WAN) links that tie together your offices. You may want to indicate that the offices are linked together by a provider such as Straitshot or Masergy.
You need to then talk about server redundancy for reliable telephony, in case a server goes down. You also should consider the method by which configuration backups are handled. Now, for outside phone service, you're either going to connect to the traditional phone network -- public switched telephone network (PSTN) -- or to Internet Telephony Service Providers (ITSPs) using Session Initiation Protocol (SIP). For the gateway, you can consider something like the MX25. This is interoperable with Asterisk and will let people choose if they want E1, Analog, or Basic Rate Interface (BRI). If you connect directly to SIP service providers, ensure that you use a SIP-aware firewall for security -- such as a SonicWALL with the latest software.
Next, consider the remote or home workers for the company. They should have a broadband connection at home. You need to have a virtual private network (VPN) concentrator at the head offices, and connect the remote home users by using a device such as the ZIP4x5 IP phone. This builds a VPN tunnel back to the concentrator and can then register with the Asterisk.
These are just some ideas and considerations to get you started.
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