With old HDD to secure delete a file you need to use Gutmann standard, but with new HDD you can use only 1 pass overwrite I'm not sure what hard drives mobile phones use and don't know how many passes will be okay for secure deleting files on phone. What standards are okay to safe delete files on phones? How many passes you need ? Is 0x00 or 0xFF standart overwriting with random data 1 pass safe or you need to use DoD 5220.22-M standard or Gutmann standart ? This question is Android and iPhones related
On modern Android and iOS devices, some type of file encryption is certainly in place. Without the encryption key, the data is irrecoverable. Therefore, performing a factory reset to remove the key should render the data irrecoverable.
This is assuming you are willing to discard all data on the device.
Mobile phones don't use Hard Drives, and rather use the flash memory chips for memory like the ones used on a Solid State Drive. So you can't use the same exact methodology that is used to wipe clean a Hard Disk Drive.
You could try factory resetting the phone, that would only "delete" the data. However, I think it would require rooting the device as a first step towards, wiping clean the data on the phone. After rooting the device you could probably do a google search on how to wipe clean a phones memory.
A better option would be to physically destroy the memory chip on the phone, in a manner that it would be hard to reassemble it back together.