Just googling for "21 Lisboa" would have found you several hits in other discussion fora. People have been reporting this cell broadcast for at least three years and it can be received in larger parts of Portugal, not just in Lisbon.
To repeat Tomas By: It is probably just a test message with no significance.
Edit: Just to add some details regarding your alleged 'official warning channel' argument. There is no standard cell broadcast channel for official warnings. The very few countries using cell broadcast messages incorporated in some kind of official warning system are all using proprietary solutions. According to the forum reports, the "21 Lisboa" message is sent on channel 50, which if used at all, in Europe is mostly used for location information.
Checking which cell broadcast services are in use in Portugal, the network operators Vodafone and MEO are also indeed using channel 50 for location information.
MEO is having or used to have different tariffs for local and long distance calls to fixed line telephones and the location broadcast on channel 50 indicated which area code was charged as a local call. 21 is btw the area code of Lisbon.