Think twice before chucking out your old telephone. Even the standard post-war black bakelite telephone (model 332) sells at auction for pounds 50- pounds 80 - the estimate on two such specimens at Bonhams Chelsea's sale of cameras, scientific instruments, et cetera on Wednesday (11am).

Telephone collecting took off with the liberalisation of the UK telephone market in 1984. Subscribers were no longer obliged to buy BT's narrow choice of models, hard-wired into the wall. Suddenly, weird and wonderful non-BT phones appeared that could be plugged and unplugged according to whim.

Some buyers at auction are trendy interior design buffs in pursuit of unusual period phones. They will pay pounds 200-pounds 300 for a white or coloured standard 332 model or for a Seventies standard model 700 - if it is transparent. Other buyers are collectors - Germans and Swiss, besides Brits, who display their finds on shelves without bothering to convert them for use. Now that it is possible to run four extensions off the same household telephone line, most of us have become collectors without realising it.

In the sale, there is a Twenties candlestick telephone (separate mouth and ear pieces) with brass trim, estimated pounds 80-pounds 120. Today's handset makes the old-fashioned candlestick - in use from 1900 into the late Twenties and beyond - look like a glitsch in the evolution of design. Did it really take a quarter of a century for designers to twig that both components could be integrated in a single handset?

The curious truth is that Alexander Graham Bell's first telephone of 1876 had a one-piece handset, (called a "butterstamp" because it looked like one). And the first telephones to become commercially successful, in the 1890s, had one-piece handsets, too. So why the cumbersome candlestick? Because it was ergonomically designed to accommodate a technological advance: a cupful of carbon granules that transduced the voice into electricity prior to transmission. If the granules were accidentally tilted, there was a roar like an avalanche and contact with the other subscriber was broken. So, until yet more technological advances, the mouthpiece was mounted on a separate, vertical column containing the granules, that had to be held upright.

The top lot of the sale is estimated pounds 2,000-pounds 3,000. It is one of three known "Gecophones" in mahogany-coloured bakelite specially commissioned in about 1933 from the GEC company for the Gleneagles Hotel in Auchterarder, Scotland. All other Gecophones, like the model T Ford, were black.