Xlink (and others) make Bluetooth bridges.
These are black boxes that contains a bluetooth interface as well as a jack to plug in a conventional analog telephone set. The blackbox generates dialtone and ring voltage, and will decode Touch-tone (acting as a mini Telephone company central office)
I use an XLink BT A12 with my Uniden wireless two-line telephone system. Line 1 goes to the local telephone company (I still have one old-fashioned twisted-pair subscriber line), and Line 2 goes to the black box.
When my cell (iPhone) rings, L2 on my Uniden handsets rings as if it was any other telephone call.
I can also dial out using the Uniden keypad and use my cell's voice dialing function.
Your milage may vary depending on your cell phone's specific bluetooth feature set, so read the specs before you order.