Nowhere in Australia has 1800MHz only GSM service. The basic coverage is 900Mhz and some areas have both. I understand Europe to be similar. The 1800Mhz band was added to increase capacity and not to operate on its own.

Mrs NM uses my old 900Mhz only GSM phone and the coverage is as good as any dual band in Australia. However, in times of heavy network congestion (say at a football grand final or airport), a single band 900MHz phone is more likely to get a "service unavailable" response. This response is due to cell congestion and not a lack of coverage.

My current phone is a Nokia 8890 whoch technically is only dual band, being 900/1900MHz. I get excellent coverage with Optus and although only operating at 900Mhz in Australia (same as the older single band phones), I rarely experience service problems due to congestion.

It is considered extremely poor form for a carrier to drop a call when you move into a new cell because that cell is congested, and they do all they can to avoid such a situation. It is better to report "service unavailable" for new calls and retain some spare capacity for cell roamers rather than drop a call during cell hand-off. However, it does happen!