EDIT: I searched and ID Mobile does not support 2g / gsm network. Writing the problem out helped me figure what to look for!
original:
I use a ID SIM card in my iPhone just fine, but when I tried it on an old Sony Ericsson T303 (released 2008) it says Invalid SIM.
Does ID mobile work on 2g network or 3G minimum?
I believe the phone is unlocked as it works with my wife’s Lebara sim (Vodafone) and the phone was originally Orange branded, now EE. And I believe if the phone was sim locked I’d be given the option of entering a code if needed.