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Connected by wifi calling, can only receive SMS from some people
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RCS based SMS work over any data connection (mobile 4G etc, or WiFi) but need both sender and recipient to have RCS enabled (it is unlikely OTP messages will use RCS)*.
This is not related to WiFi calling/SMS over WiFi. Think of it more like Whatapp, or Facebook messages, but that display in your SMS app.
Proper SMS over Wifi, which it seems O2 haven't fully rolled out, once working allows normal SMS sent over the mobile network to be received over a WiFi network (the mobile network provider does the clever bit).
So with working SMS over Wifi, any SMS messages should work.
RCS messages are likely to be the ones currently being received, when there is no mobile network.
I suggested testing your 1p SIM in your wife's mobile, just to make sure the A12 works with 1p's implementation of SMS over WiFi, in case it is an issue with the device, rather than the network provider.
*ETA: it also requires the mobile network provider to support RCS
When the sender sends an RCS SMS and the recipient isn't using RCS, does it resend as a standard SMS? If so, would it not be better if all OTP messages were sent using RCS so that those with no mobile signal might receive them regardless? Or are there security concerns when using RCS?
I suspect many OTP messages don't use RCS due to old technology used for the SMS sending service. Adding RCS also would add complexity and processing, whereas traditional SMS (from the sender) is fire and forget. RCS requires retries, and failover methodology at the client side.
RCS is also Android only, on supported handsets, where enabled, so limited base.
As there is a general move away from SMS for 2FA,/OTP I suspect adoption of RCS for these will be minimal.
And the OP issue seems to be the opposite, with RCS getting through on WiFi, but not normal SMS (seemingly due to lack of O2 SMS over WiFi).
Unfortunately o2 made a complete mess of the implementation and for years had no SMS support, it's only recently started to integrate SMS but it's still a lottery if the handset can use it.
Dont get confused by RCS, it's essentially another version of Apple iMessage but for Android phones, if the sender and receiver have RCS capable handsets and the network supports it then those text messages are sent as data (like iMessage does) and don't rely on network signal. It is just a bit of a confusing red herring in a conversation about Wificalling.
Why is it this sort of problem is not easy to find when choosing a provider?
P.S we both chose O2 nearly 20 years ago when they were the only one covering our house, and then wifi calling did not exist. Now we have lost the O2 signal but in the mean time a couple of years ago EE built the new mast that now covers us.