I'm not surprised the phone number for AIS doesn't work, AIS moved out of Heathrow some time ago!! Try 01489 612500 and ask for AIS or FIR.
By the way, the circle round Oakhanger is misleading, it only transmits towards geostationary satellites on the equator and according to RAF documents, is only hazardous to some aircraft systems (not to humans) south of an east-west line through the site.