[Dave Moll]

It may be that your wireless extension is not providing third-wire ringing, as expected by a "converted" 706. There are two simple solutions:
1) If you have a spare ADSL (broadband) plug-in filter, the UK ones include a ringing capacitor to provide the desired third-wire ringing. Plug this between the output of the wireless extension and the 706.
2) Failing this, you could "unconvert" the 706 by removing the blue wire from the terminal T5 in the 706 and parking it on an unused terminal such as T13 (assuming that green is parked on T14), then ensuring that T5, T6 and T7 are connected together and that T8 and T9 are connected to each other. The telephone will then use its own internal ringing capacitor.