Some towns in Portugal have not 1800MHz network

[inquisitor]

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