Lots of people are looking for a better deal or to switch carrier with their service provider everyday - this can be for a variety of reasons - poor service, coverage or just ready for a change or to save money.
If you are still in contract and have a number of months left, you will be responsible to pay the balance off in order to leave that carrier, Sprint in this case. These are your early termination fees - basically you must pay out the remaining months of payments on the contract or phone finance agreement.
Not under contract but with phone finance
The same as under contract - you are liable to pay the remaining finance payments for the phone that you have. For Sprint you may have a lease agreement - this is like a rental agreement, even when you have completed the term (usually 24 months) you still don't own the phone
You would need to pay all monthly lease payments that are remaining and then hand the phone in good working order, back to Sprint.
OR you can pay a final device payment / option to purchase - that would allow you to keep the phone
To avoid Early Termination Fees - there are currently three carriers that will offer to pay your Early Termination Fees and give you up to $650 for each phone you hand in. The carriers who currently offer this promotion are T-Mobile, Verizon, and AT&T.
Remember however that the payments or incentives that you receive for switching to another carrier may not be paid in full in one go. You would need to pay your own ETF with your current carrier and then switch - then you will start to receive the cashback. This will be split between upfront payments via prepaid cards or bill credits
Usually you have to trade in your existing phone to get the full amount of the "upto $650" offered
There are a three ways to approach this matter
You can continue with your monthly service plan and phone payments
You can attempt to have another carrier pay your Early Termination Fees for switching
You can cancel your contract and pay the Early Termination Fees, relinquishing you from the contract