Free calls

You could use Skype to call between any PC local or worldwide (also conference calls) for free.
You only need a mic & speakers or a headset also you can make video call with a web cam.
USB phones can also be used to have a landline look & feel.

http://www.skype.com/en/

http://dx.com/c/computers-networking-399/networking-314/skype-phones-338

Phone-line networking:
The technology used to make it happen.
The advantages and disadvantages of using a phone-line network.

http://www.howstuffworks.com/phone-network.htm

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You could use a PBX system like Free PBX on the same lan and make calls using your own software. if internet went down you don't have to worry about your internal call dropping.

http://www.freepbx.org/freepbx-distro

there is plenty of help to get this running. It is a basic one run cd or you can use VMWARE if you want if you don't have enough computers. Be sure that the computer with vmware will have enough ram.Example would be giving 640 ram to the vm and having at least 4 gigs on the host computer. using the vm will make a ZERO investment in equipment other then your time to set up the system. the link also has a few quick start guides to help things along. OF COURSE you always have us!

You could also use free wifi calling android apps:
http://reviews.cnet.com/8301-19736_7-57566261-251/five-great-android-wi-fi-calling-apps/

Thanks TechGua,

Very interested but I couldn't find requirements and deployment guide.

Thanks

FreePBX is not free.
Read the details in the link given.

Skype offers completely free calls between any PC or any skype installed WiFi phone including all smart phones, android, iphones.

Free pbx is free....they also have paid versions...I have freepbx on my network and it cost me nothing...works great...pbx in a flash is the second distro I supplied is absolutely free. Skype is paid for if ou want special features. ..just like any service...but if your internet goes out.. your skyoe calls will be dropped, where a internal pbx server will keep the call going.  Since sype relies on your internet it is not used by bussiness for internal communication (there are some exceptions) but the fact remmains...no internet no skype.

It does not connect to smart wifi phones so it is not free in the sense that you need to buy USB phones or headsets and web cams for all PCs  to make calls (modern laptops have a mic, speakers, and web cams built in )

I dont think you know enough about pbx systems. And thats why your saying skype...why not yahoo ?? First of all , all smart phones can do whats called SIP. So connecting your phone to your server is a snap. Skype also needs usb head sets .so your point of saying that this is not free on bounds that the sysem needs it.. is true with any system needing mic and speakers..well so does skype. Another important factor is that this is a business. With a pbx calls can come on and ring every one or just the people you say. The features of a pbx system are a 1000 fold compired to skype. Not to mention the voice virtual attendent to direct calls to an extention , voicemail, email notification. Etc....a full solution.

The question states:
"if possible can use the normal landline"

text, etc is not needed as per the question.
But additional phone or hardware purchase expense is not desired by the wording of the question.

Most, if not all of the staff will own a smart phone, thus a one on one call as asked for in the question " if possible can use the normal landline"
Inclines me think that skype is the best solution.

The internet being out of service or down is seldom.
On the other hand, the buildings power outage would ruin your advised PBX idea, but smart phones would not be affected.  
(given that a wifi connection should be available from an another source)

I ask the owner of this question to please state the type of system you would like. A internal phone system or skype. (Skype can not connect land line to share with all his emploees , only a pbx system can do that)

https://www.experts-exchange.com/Networking/Telecommunications/IP_Telephony/VoIP/A_10294-SIP-VOIP.html
http://www.asterisk.org/
http://www.asterisk.org/products/telephony-interface-cards

i want to further add that if said person wanted, a Google voice account could be created for free with a free  number, then their existing phone could be forwarded to that Google number. After, incoming calls could be shared through ring groups on a pbx system with Google voice trunk set up, connected to each smart phone or iphone or ip phone/softphone, and if no one answered, it could even send a call to a cell phone or several at the same time.  I will help you set that system up, its not hard.

Is the OP still here?