It depends if your old phone is network locked. There's a simple test you can try to check. Insert a Vodafone SIM card into the device and if it still works then it's not locked.
If it doesn't recognise the SIM card or asks you for an 'unlock code' then you will need to contact your old service provider to request a network unlock code. Once you get this code follow the steps provided to unlock the phone. After that just pop a Vodafone SIM card into the phone and you can start making calls & texts.