The advantage of the dual SIM phone is that one handset can operate with two SIM cards.
Dual SIM handsets are used by different groups of people, and here are three possible groups.
Business people. Dual SIM phones allow business people to use the same handset for personal and business purposes.
Travelers. Dual SIM phones allow travelers to use the same handset with the SIM card of home country and the SIM card of the country they visit.
People living abroad. Dual SIM phones allow people living abroad to use the same handset with the SIM card of home country and the SIM card of the country they live in.
A very important point is that many dual SIM handsets can operate only in a limited way. These handsets include the Lumia phones.
When two SIM cards are used, only the one that first connects to the network can operate on 3G and 4G connection speed. The other SIM can operate on 2G connection speed only.
This is not a bug instead it’s a known technical limitation of the design.
People wanting to use their handset in a country where 2G network is no longer available are the most impacted by this limitation.
The shutting down of 2G networks is something that is happening. Recently, Singapore has finished shutting their 2G networks down in April 2017. That means dual SIM Lumia phones can operate with single SIM only.
Other countries such as Australia, Canada and United States already has begun planning for shutting down their 2G networks.
It’s an observation that in Singapore dual SIM models are not available for purchase directly from the vendor Microsoft. Customers either need to go for the single SIM model or for an export set.
Business people tend to travel into different countries, or even permanently stay abroad. They want to use their dual SIM phone in a way dual SIM phone should be used. In the upcoming premium device, whatever it will be called, it’s a must to have this limitation resolved to allow simultaneous connection via 4G for two SIM cards.
Currently, it seems all the dual SIM Lumia models affected by this limitation including Lumia 640 and Lumia 950.