Hello
Nice to see someone from gsm. i also spend more than 15 year in gsm reverse engineering & programming.
Infineon & locosto cpu base phone made by sagem, old time we used jtag to read memory dump and decrypt data for unlock code.

Most secure in old time and never cracked security publicly is by blackberry (company is dead long go) qualcomm cpu base phones, same principal use in latest modern hardware wallets.

Its not secure to relays on any older security. for example RSA 1024 its still not broken but consider unsafe everyone move to RSA 2048.

There is always EMMC/JTAG method to dump data on old phones. some may be encrypted but only firmware part data is decrypted most case.

I think latest qualcomm & samsung trustzone and trusttonic are best to start, it may be costly but security never come in cheap.

samsung exynos & qualcomm latest phones are more secure currently for research part for wallet.