This book describes how you can turn a Raspberry Pi into a business class fully functional VOIP PBX. The system is capable of making and receiving calls over GSM and the internet to and from all landline and mobile numbers. By using a DECT telephone system like the Gigaset A580 IP and a 4G Router like the Huawei LTE CPE B315, the telephone system needs no landline or telephone cabling. It is almost wireless which makes it an ideal solution for temporary offices etc. Detailed instructions are included to guide the reader through the installation and configuration of all the relevant hardware and software.