Several top providers currently offer incentives to help new customers transition from their previous carriers:

Provides a buyout of up to $2,500 ($500 per line for up to 5 lines) in the form of a Visa® Reward Card to cover combined device balances.

Get $2,500 to Switch Carriers | Contract Buyout Offer - Optimum

Offers switching deals that can provide up to $800 per line to break a contract and pay off phone balances when you join certain plans.

Offers a popular "Keep and Switch" program that reimburses up to $800 per line (for up to 4 lines) for customers switching from eligible postpaid carriers like AT&T or Verizon. They also have "Carrier Freedom" deals that pay off ETFs and phone balances for those who choose to buy a new device from them.