JioPhones comes with 4GB Internal Memory - so just like a normal Android smartphone, you can save as many contacts depending on the free space in your phone. In practial sense, you can store unlimited contacts.
No. It is mandatory that you have to use Jio sim card in JioPhone.
Unlimited. JioPhone comes with 4GB Internal Memory - so, you can store as many contacts as you want, depending on the free memory on the phone.
Yes, JioPhone does have a 3.5 mm headphone jack.