Verdict: True

YUP. According to The Economist, it was an advertisement for dentistry. “Messrs Gabriel, of 27 Harley Street, advised that their dental practice would be open from 10am to 5pm until October.”

We've had to deal with this nonsense for 150 years and it's as annoying now as it was then. Time also verifies this, though not the specific example given by The Economist: “the first unsolicited messages came over the wires as early as 1864, when telegraph lines were used to send dubious investment offers to wealthy Americans.” It appears that people and institutions would in fact receive unsolicited telegrams.