Most smartphones lately support more than one SIM card with means you can use your device to connect to two different networks simultaneously. Some phone makers support physical SIM cards and others have support for both physical and virtual cards called e-SIMs.
ALSO READ: eSIM cards explained
One would choose to have one network for calls and the other for data mainly for financial reasons. The exact support for each SIM slot depends on the phone but in most cases:
- SIM1 supports all features of the network (2G/3G/4G/data)
- SIM2 can be used only to make or receive voice calls, in many cases 2G only
For example, if I find that Africell or Airtel give me a good data bundle rate, I will opt for that to be in my SIM1 slot and maybe MTN in the SIM2 Slot for affordable voice rates. Since modern phones are to small to have the filters required to operate radios simultaneous in transmit and receive mode (full-duplex) the main Tx/Rx radio performs registration, calls and SMS for both SIMs but not at the same time.
While not in call primary radio transfers data and listens for incoming calls or SMS for SIM1 while the secondary receive-only radio listens for incoming calls or SMS for SIM2.
While the primary radio is in a call for any SIM the other radio is suspended. This happens because networks number 1 and number 2 not being synchronized it is impossible to make sure half-duplex operation is possible between the two radios.
In some cases there is no difference between SIM slot 1 and slot 2 — these are known as Hybrid SIM slots, both behave in the same way, but in most of the cases, there is a difference. The difference is that only one of the slots supports LTE and VoLTE (Voice over 4G), which is the reason the companies are asking you to use the SIM card in slot 1 as in most cases slot 1 has the LTE/VoLTE support.
You check this by putting for example a Smile Telecom SIM in slot 2, it won’t show any network as Smile is only 4G and there is no 3G or 2G coverage, but if you put Airtel sim in slot 2 you may get network as Airtel has 2G/3G support too.
Most people using their primary SIM1 frequently for voice, data, and for everything. Secondary SIM2 is for either voice or data or for some network issues. Hence mobile manufacturers make the mobile by single SIM slot can do all functions at a time.
Some more powerful phones lately, there is no difference between slot 1 and slot 2 both come with the same capabilities. Both slots work the same. Case in point my Galaxy s21 Ultra has support for all networks for both SIM as shown in the image above but only SIM 1 has support for all 5G/4G/3G/2G networks. Of course, we have no network in Uganda that supports 5G currently.
But if we talk about old phones there was a difference between slot 1 and slot 2. Slot 1 always comes with more features than the 2nd slot. Like in some phones slot 1 provides 5G/4G/3G/2G and slot 2 only provides 2G.
The question come back to you, Which network is worth being your SIM1? Leave us a comment below or on our social media pages.
