I retired from the wired telecommunications industry just as my company was deploying Cisco phones. Making phone calls over a data connection without QOS (quality of service) on it could be problematic as in dropped packets or getting received in the wrong order (both ends). When the data bandwidth is plentiful it should not be a problem. It's when the towers are congested or a weak signal is when you will have problems. I guess you can't make the calls with your cell phone and a bluetooth headset as you need the Caller ID on the far end to reflect the company number and name. I wish I knew more about how cell phones work. If it was the other way around, the Cisco phone could "spoof" your cell phone number. Your IT department could do that. Be sure to get a signal booster with a good directional antenna. I guess you could park by a library and use the WiFi repeater to connect to the library's WiFi (or other WiFi). Libraries normally have a good internet connection.
