Can I use my iMac OS X Yosemite as a phone and phone system
Yes you can do just that. You could use an SIP line. There are hundreds of phone companies which offer SIP phone services (also called VoIP). Some internet provider also give you the access to the SIP configuration so you can handle your calls the way you like.

Check with your provider if they use SIP for your phone line and if so they could actually let you manage your phone calls with your own software.

Then you would have to install one of the SIP apps to handle your calls and even configure the way your calls are redirected or answered depending on the time of day and other parameters.

A list of SIP server software is available on wikipedia. Asterisk is the most common one is free and quite full-featured. There is a host of visual interfaces to help you with the administration and operations of SIP lines through asterisk, you can also find entire distributions dedicated to asterisk.



If installing and configuring your own SIP setup sounds like too much trouble some providers will have many features like:


multiple phone numbers
multiple phone calls per number (incoming and/or outgoing)
auto forward
redirection after x tones
call transfer
conferencing
call monitoring
welcome menu: "Welcome to My Global Company. For department press 1..."
auto attendant: "If you know the extension of the person you would like to speak to, you can enter it now"
announcements: "Welcome to MGC. Please wait while we connect your call"


The level of service depends on the provider you chose. But the advantage of using a standard VoIP service is that you can move your setup to a new company, add lines from multiple companies and do whatever you like if you setup your PBX yourself.



If you want to use a normal phone there are SIP converters which can be installed between your internet provider's router and your phone. There are also full featured SIP phones.

Some phones let you use both, your normal land line and multiple SIP accounts, for example I use one similar to this one:


Other phones have a phone standard integrated which let you setup call redirections and other features.



There is also skype as already answered here.

They offer a very simple package but it is severely limited in comparison to an SIP installation.

There are multiple phones available to use skype like a normal land line (note: these are not compatible with the standard SIP protocol)

You can subscribe to a skype number to receive standard phone calls directly to your skype account. So people will be able to call that number without having to use skype on their end.

But when it comes to managing multiple lines or setups with multiple extensions skype is not made fo that.