I would be shocked if people couldn't find an RCE in an early 2000s flip phone. I had a friend who had hers since 2010 and MMS crashed it all the time.
If you're talking about flip phones and trying to protect against an eavesdropper of a phone call or SMS, then there's no point. The network that these phones used is full of holes already https://en.wikipedia.org/wiki/Signalling_System_No._7#Protoc...
Attack surface reduction is the important part. I’m not in disagreement with what you said, but if you took a modern iPhone and removed all capabilities other than sending and receiving phone calls, it would be much more secure than one which supports mms, email, browsing, etc.
Are you sure having a phone in your phone is a good idea? Phone calls are a significant source of attacks now, even if none of those attacks exploit a vulnerability in the phone software. As far as I'm concerned, the only point in having a dial-able phone number is to ensure I'm still eligible for car warranty scams and 2FA code harvesting attacks.
