Simply put, dual SIM phones are handsets that hold two SIM cards. Initially, dual SIM adapters were made available that enabled people to switch between two SIMs housed in their mobile phone.

More recently however, manufacturers have made phones which allow both numbers to be active at the same time. These are known as activedual-SIM phones.

The bonus of a dual-SIM function is that you can maintain both a work number and personal number on the same phone – rather than carrying around two handsets. 

Additionally, people regularly travelling between two regions may save considerable expensive from roaming charges by keeping a second SIM for their destination country, thereby taking advantage of domestic charges.