I will need a different sim from Plusnet
Why will you need a different SIM , the SIMs supplied by PlusNet have pressouts for Mini,Micro and Nano sims.
Doesnt your iPhone need a nano sim ?, in which case , if your Xperia doesnt already use nano sim, then just press out the nano sim from your existing sim.
If your iPhone uses an Esim then you've got a problem I'm afraid, PN don't currently support Esim
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