New phone - with someone else's data on it

[KeenReader]

Query about serious breach of customer data protection.

Today I picked up my first smartphone & in theory had my number & data transferred over from my old vodafone nokia. When I got home I was horrified to find I had someone else's data on it ie all their texts & contact list. Has this happened to anyone else? On return, the store staff deleted them & put my own contact details on. They said it was "impossible" that my information was on anyone else's phone but could not explain how I had got a Michael's data. Does this mean that I have been sold a second hand or stolen phone? I ordered it online for collection in store. The staff opened the box so I don't know if it was sealed. I am very, very concerned that all my data is on a stranger's phone, I tried to resolve this via live chat with no success. Any suggestions on how to proceed to protect myself? 

[BandOfBrothers]

You have 14 days cooling off period from the date you picked it up to return the phone and exchange it out. I appreciate their is not possibly something wrong with the phone but your gut feeling is altering you. 

If this was me I'd be swapping it out and ensure the box is sealed. 

[kids]

To address your question.

As it has happened to you it is not beyond the realms of possibly that your data could end up on someone's else's phone.

I would change the passwords on any of the apps on your phone such as emails just to be on the safe side and keep a watchful eye open for any unusual activity on any accounts that are associated with your phone.