There are various levels of restriction on the use of GSM gateways, in different parts of the world, because they waste scarce air interface channels in getting traffic into a a fixed network when it could have been put directly into a fixed network.
Asterisk can be used to make calls to cellular phones by (in approximate increasing order of being restricted by law):
connecting to the normal PSTN network with analogue or digital lines, and through that to the cellular network;
connecting to the PSTN via VoIP over a data network to an ITSP, who then does (1);
connecting to the local cellular network directly over its air interface, using an cellular gateway device, fitted with a SIM card;
connected to local cellular devices using open source software and hardware that implements a private cellular base station.
(1) to (3) always involve paying a service provider. (4) will generally require paying for a, government issued, radio transmitting license, and such licenses may be impossible to obtain in many parts of the world, by anyone other than a national cellular operator.
More details on external connectivity can be found in http://www.asteriskdocs.org/en/3rd_Edition/asterisk-book-html-chunk/asterisk-OutsideConn.html