Well, if you're on a tight budget a 1 line system can get the job done just fine. Person calls in, IVM answers their call and asks for the extension (or to 'dial by name') and then IVM transfers the call over to the appropriate extension.
For this set-up I recommend using a VoIP system with a PBX (e.g. NCH's free Axon PBX software). The PBX will manage your calls, and route them to the appropriate destinations, and handle call transfers and voicemail. The basic concept is that each employee has an SIP compatible VoIP phone with their own extension. Each one of these phones is connected to Axon via your network, and each extension has its own corresponding mailbox created in IVM. When a call comes in, Axon sends it to IVM, IVM determines where the call is destined for and transfers the call there. If there is no answer, Axon will send the call back to IVM which will then take a voice mail message.
In terms of actually hooking up your system to a phone line people can call in on, you have three options:
1) The VoIP method:
Pros:
- Much cheaper, often around $5 - $7/mo for incoming phone number as opposed to normal landline providers that charge you on average in excess of $25/mo
- Cheaper outbound rates as well (many VoIP services offer great rates like 1.5c/min to anywhere in North America, $35/mo for unlimited worldwide calling)
- Doesn't require the purchase of expensive professional Telephony Boards or other hardware
Cons:
- Requires a reliable and relatively fast internet connection (all calls put through over the internet)
- Voice quality varies based on network set-up, distance factors, interference, other activity on your network, etc.
2) The Telephony Board method:
Pros:
- No Internet connection required
- Relatively easy to set-up if you already have a normal phone service running at the location
Cons:
- Boards are costly (most in excess of $200)
- Requires normal land-line phone service (more expensive than VoIP)
3) The FXO adapter method:
Pros:
- No Internet connection required
- Cheaper than a telephony board
Cons:
- Quite difficult to set-up, may require the help of an expert
- Can only support 1 line per adapter, as opposed to telephony board which could support several lines
- Requires regular phone service (more expensive than VoIP)
If you go the VoIP method, it may be cost effective to just give everyone a separate number considering the fact it only costs around $6 or so whereas a normal landline telco would probably charge you $50 start-up fee for each line as well as an ongoing monthly of fee of $25+