VOIP Phone Systems, Truly Mobile
A VOIP telephone system is hosted in the cloud and provides all the same features of a traditional on-premise PBX type telephone system.
With a VOIP hosted telephone system there are no expensive costs for things like ISDN lines or for maintaining old phone system equipment.
A VOIP telephone extension merely needs an Internet connection to sign-in to the service to start making & receiving calls, so you could plug in anywhere and be an extension on your phone system.
Start with just 1 handset and scale up to 100s. Use a soft-phone and make/receive calls using your computer.