It is possible to use your cell phone as a VoIP phone through various apps. CSipSimple is an Android based app that enables cell phones to handle VoIP calls. Whenever a VoIP call is made, an answering screen comes in that is distinguishable from the normal calls. The dialer for VIP calls is also different from the normal calls.
The setup process is simple and straight forward. It is recommended that you use the basic setup and configure the device to only be available on the WIFI network. This means that the device automatically picks the local network whenever it becomes available. Whenever you are in an area without WIFI, all calls go to voice mail. This ensures that you are not disturbed by office calls while you are at home.
On an iPhone, you may use Zoiper, which has equally good functionality. The app is also available on android in case you need to standardize the software on different applications. The applications provide you with a convenient and unique opportunity to with workmates within the office network.