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You could port your phone number to a voip service and just set it to forward to the new phone. That would run about $25 / month. You could even call out on that number using the voip app.
Do
any VOIP services handle SMS
and MMS messages properly?
I am also interested in prying my phone numbers away from my mobile provider, but who to transfer
to is the harder question, when both SMS and MMS are important.
Note: When both sender and recipient(s) of a text/multimedia message are using Apple devices
and have allowed the feature, the messages are in fact transfered using iMessage and normal mobile data, not the mobile carrier's SMS system.