Some towns in Portugal have not 1800MHz network
 

I'm now in Portugal, especifically in Peso Da Regua, near to Porto. I have a triband (850,1800,1900) Nokia phone, that works without problem in Lisboa, how ever here i have no service. I tried a four bands phone and i could detect all networks that works in Portugal (TMN, Optimus and Vodafone).
So, here only are 900 MHz network and maybe 2100 for 3G/UTMS (i don't have how to test).

That's normal. As far as operators have licenses for both frequencies, 900 and 1800 MHz (which is the case for all Portugese operators), they will always prefer 900 MHz over 1800 MHz, since the former has nearly double the range of 1800 MHz and so they need less base stations for the same coverage footprint. 1800 MHz is only used to expand capacity in urban areas or wherever necessary.
In other European countries there are operators, who have only 1800 MHz spectrum - there you would experience comprehensive coverage with your phone, as they don't have the alternative of going 900 MHz.

Really? Which countries in Europe use only 1800 MHz? I was under the impression that GSM was the standard there since the early 1990s before UMTS came along and that GSM started out as 900 MHz only, with 1800 being added later on.

T-Mobile and Orange in UK both are 1800MHz only.
In Italy and Germany also Wind, E-Plus and O2 were 1800MHz only, now they have also EGSM900. I think also T-Mobile in the Netherlands should be 1800 only as well as Orange in Austria (now with some EGSM 900 too).

I think GSM has been 900/1800 from the outset, with competitive licenses being awarded in either band.

No, GSM 900 was introduced in 1991, while the 1800 MHz band was specified 3 years later. Until 1997 there were no dualband phones, that would have supported both frequencies, at all. Further those first dualband phones were not able to perform an inter-band handover, but users had to switch bands manually.
So it would have been useless for a network operators to roll out networks on both frequencies, especially since handset manufacturers regarded dualband phones as "some kind of exclusive, costly, solution", that was unlikely to become reality berfore 1997.
Actually we discussed that recently at http://www.prepaidgsm.net/forum/off-...html#post29176

Bouygues Telecom was a only 1800 net...