Dual Sim.
Dual Sim is a Facility to have 2 GSM phone numbers in one SIM. To use this facility your phone must be a Dual SIM compatible phone, the way to support this Line One, Line Two Facility. The user can get the Phone number from the service provider. Thus allowing the customer to be reached via the Line Two if it is not possible to reach through Line one. This is also called as Universal SIM card (to hold two SIM's in one).
The latest development allows you to have both SIM's are to be active to receive calls and to receive SMS and MMS in your mobile phone.
And this mobile phone is called as dual SIM standby mobile phones. On the other hand the primary number will allow you to call numbers and allow you to send SMS and mms to other phones.
Nearly all the mobile phone companies have manufactured and are distributing these kind of DUAL SIM ST.BY mobile phones all over the world. Many carriers too are selling these phones with different plans to their customers at low price.
If you are interested in buying such a phone first try with your carrier first and then count on your friends mobile phone numbers which are connected to which carrier. Then buy 2 SIM card from those carriers which of your friends mostly have their accounts. So it will be cheap for them to call you.
Like wise make the SIM card of the carrier as the primary number in which your friends have most connected by their mobile phone.