I want to buy new phone but I am unable to understand the meaning of 実質負担金額.
Please help me to understand this.
If you break it down into 実質、負担、金額...
実質 is 'real', 'actual', etc.
負担 is your burden, as in to bear a cost.
金額 is an amount of money.
So, the actual cost you will bear, the amount you're actually going to have to pay.
If you have a look, it looks like there's a set cost for the phone, but if you take away the monthly discount (24 months worth), this is the amount you will "actually" end up paying.