Strange behaviour of the double SIM in Japan


I’m currently in Japan and got a local mobile contract through Y!mobile (which, as far as I know, uses the Softbank network). However, I’d also like to continue using my Finnish SIM (mainly for receiving and sending SMS back home), and the double SIM feature seems to suddenly be working quite strangely: only the SIM that is currently 3G enabled connects to the network - and I mean the phone network. I mean it would be quite understandable if there’s no data connection, but the SIM that’s not 3G enabled doesn’t find the network at all - it can’t be used even for calls or SMS! When my Finnish SIM is 3G enabled, it finds the NTT docomo network and can be used for calls and SMS, but if it isn’t 3G enabled, it doesn’t! And the same goes for the Japanese one wrt Y!mobile network. Before, when I have been travelling, I have been able to use two SIMs at the same time with no problem.

Do you have any idea on why this happens and if there is a way to fix it? Grateful for your answers! :slight_smile:

Using the forum search, you’ll find several previous threads about incompatibility issues of the Fairphone 1 in Japan, for example this one. I haven’t delved too deep into it, but it seems you will not be able to connect using a Japanese SIM. :frowning:

It appears SoftBank / YMobile does not offer any 2G services. Hence with 3G not enabled your phone will fail to connect with the YMobile sim.

According to, other operators are not offering GSM either so no roaming on a GSM network for your Finnish sim, only on 3G.

Edit: indeed states, there is no GSM in Japan.

But my phone DOES connect using the Japanese SIM. I can use data and everything through their 3G network. The only thing is that I can only use one SIM at a time.

Interesting! So this means that calls and SMS also go somehow via 3G? Anyway, thank you for your answer!

Yes, you can have phonecalls over 3G (UMTS) networks (or 4G LTE, on the FP2). That is how it works in other countries as well, your phone does not switch to a different radio frequency just for making calls.

Due to the architecture of dual-sim in the FP1 (and I think in the FP2 as well, will test it in 5 weeks), only 1 SIM is usable in Japan, the one with 3G selected. There are no 2G networks in Japan. You can switch 3G between SIM cards and use either one, but that is not practical.

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