How do I clear all personal data, before returning the phone?
Many Mobile Telephones give an option to record personal numbers etc. into the phone memory or onto the SIM card. The SIM card often has more memory and this should be the chosen option. If you remove your SIM you should be able to access the telephone memory and see what is in it. Delete all the personal information, there should not be much on it. If you find that all your personal data is saved to the telephone, you should be able to save the information to the SIM, then remove the SIM and delete the data on the phone. If you are keeping the same number and same SIM, then your personal data will be available on your new phone. If you are changing your service provider and getting a new phone, by putting your old SIM into the new phone you can transfer the SIM data to the new phone and then onto the new SIM. After you should destroy the old SIM by cutting it up through the Electronic Chip. If you have to send it back send it in pieces.
The more modern android type phone will have an option to connect with a PC to recover your data. Alternatively, you might be able to email the data to yourself. After which you will need to either delete all personal stuff if you don't have a memory card in the phone. If you have a memory card just remove it and install it into your new phone, all your data should be there.