The national geography has released a phone with two sim cards slot meaning it is a dual sim phone. It was previously announced is now released and available in the markets. Around 185 countries worldwide will be made accessible to these new products.
few specifications of the phone are:
Tri-band GSM (900/1800/1900 MHz)
Other features include a 2.8-inch QVGA display, a mobile TV, Bucktooth, an FM radio, a VGA camera with video recording, a 1GB MicroSD card, and a pre-loaded National Geographic content (ring tones, videos, wallpapers)
This phone is not locked to a specific network therefore if you are a frequent traveler and need to travel to other countries you can use this phone in the other country as long as you get the sim card for a mobile phone service provider in the current country you are at. I think roaming applies to this principle you can see your current provider and ask about roaming coverage and charges in other countries.
The beauty of having a dual sim phone is you can have two sim cards for different service providers example you can have a sim card for AT&T and Sprint both in one phone this is excellent, you can benefit in phone plans for either providers not only that but if both have either phone providers say launches a promotion for say - free sms or free calls you can benefit.