Here is the procedure I use for a lab on key system PBXs that require access to an analog phone line. It uses a Bluetooth Cellular Gateway, which is very low cost.

The last method, which we will use here, is to insert a Bluetooth Cellular Gateway between the PBX and any cellular telephone. For example, an XLink BTTN can be used as seen below with the PBX in place of the telephone on the right.

In this type of connection attach the cellular gateway’s top RJ-11 port – DO NOT USE THE BOTTOM PORT AND DO NOT CONNECT THIS DEVICE TO AN ACTIVE PSTN ANALOG TELEPHONE DEMARC - to the CO1 port on the PBX using a standard analog PSTN telephone cable. Power up the cellular gateway by connecting it to electrical power. There is no power switch. Pair the cellular gateway to a Bluetooth enabled cell phone by pressing the button with one dot beside it until the button begins to flash, then activate the Bluetooth pairing function on the cell phone. When the two devices find each other, complete the pairing as required by the cell phone. If the cell phone requires a pairing code use 0000.