Unfortunately, it's not possible. (It's a bit like expecting the radio in your living room to send your voice back to the BBC. The circuitry simply doesn't allow it).
If you've both got mobile phones (on tariffs which don't cost an arm and a leg) the solution seems obvious. Otherwise the cheapest wireless solution would be to buy a pair of walkie talkies. The really cheap ones (around �15 per pair) can get through batteries at an alarming rate. It would be better to buy a pair which come with a base unit for charging the built-in rechargeable batteries. They're available from Maplins, Argos or larger branches of Tesco for around �20.
Chris