How to Connect an iPhone to a Landlineby Daniel Hatter
One of the features included with many newer landline phones is "Connect to Cell" -- also called "Link to Cell" by some companies -- which enables you to connect your landline to your cell phone via Bluetooth technology. The iPhone 3GS, 4, 4S and 5 all support Bluetooth, so as long as you have a relatively recent iPhone and a landline phone that supports cell phone linking, you can connect them. Once connected, you'll be able to make and receive calls on your iPhone using your home phone handset.
Press and hold the "Bluetooth 1" (or similarly named) button on your landline phone's base until it starts blinking.
Place your iPhone somewhere within 10 feet of your landline phone's base and open the "Settings" app on your iPhone. Tap the "Bluetooth" setting and then tap your landline phone's name in the list of devices.
Enter the landline phone's PIN when prompted by your iPhone. On most landline phones, the default PIN is "0000." Your iPhone will now be connected to your landline phone system. To place an iPhone call using your landline phone, press the "Cell" button on the landline phone, select your iPhone, enter in the desired phone number and then press "Cell" again to dial.