There are devices available, a lot of office blocks need them as they are made largely of steel and glass which are not great for 4/5G networks. The devices that I've seen are manufactured by Kymata and the UK distributor is Cogent Distribution in Newbury, not sure of price but they won't be cheap.
You'd probably be better off getting a decent broadband connection and set up a wifi network with repeaters (if necessary) to ensure good internal coverage. If you want voice comms then it is possible to get an IP Soft client (phone) on a computer (if you do this then invest in a suitable headset) or mobile device or you can simply use the calls function in WhatsApp (or similar)