phone heating up

On the 15 February I purchased a Nokia Lumia from jet and after 3 days I had to return it,reason being the phone was heating up.When I took it back I explained to jet that the phone was heating up within 5 minutes when on a call so the sales assistant gave me a new Nokia Lumia and now my phone is just 10 days and ita heating up.This is the second Lumia that's heating up.Its ridiculous for a 10 days old phone to be sent for a repair.I purchased a blackberry from game and I returned it.I thought maybe if I purchased it from jet or edgars I wouldn't have problems like this.i am extremely disappointed with the jet and Vodacom.i actually changed from cell c to Vodacom to use my Lumia and experience was terrible.I even told the sales assistant I do not want a refund but just a good phone.I need to have my phone with me all the time because of work ,i cant even be on a call for 5 minutes cos the phone gets so hot.I have to switch the phone off and take out the battery for the phone to get cool. EXTREMELY DISAPPOINTED CUSTOMER.MRS NASEEMA CARIM