I was very worried too, but signal issues with Three, Vodafone and O2 where I live meant that moving to an EE backed network was the only option for me. I moved from Three to spusu at the beginning of the month (with a number port) and I appear to be getting OTP texts. Well kind of. Some appear to arrive instantly while others take a bit more time. And for a recent application with HSBC, it never arrived (after multiple attempts). But tried the next day and it arrived instantly. I don't know what is going on, but it could be something at HSBC's end. But so far, nothing disastrous.
The porting itself - well, that has been a messy affair so far. Despite my number having ported "successfully", my Three eSIM still appears to be active. spusu are looking into it but they blame Three. Spoke to Three and guess what - they blame spusu. So now I'm getting charged by both of them
And spusu support hasn't exactly been great.
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Weirdly, if I make a call with the Three sim, the caller ID is still my old number. But incoming calls arrive on the spusu number. It's a right mess.