Two years after AT&T was forced to abandon its bid to purchase T-Mobile US comes another sign of the obvious: having more carriers makes the wireless market more competitive.

Today, AT&T announced that it will give up to $450 worth of credits to T-Mobile customers who abandon the carrier and switch to AT&T. The incentive has two parts: a customer would trade a T-Mobile smartphone for a credit of up to $250 "which can be used toward AT&T products and services."

"Trade-in values will vary based on make, model, and age of the smartphone, but many of the latest and most popular smartphones will qualify for a value of $250," AT&T said.

Second, T-Mobile customers can get another $200 credit per line "when they transfer their wireless service to AT&T and choose an AT&T Next plan, buy a device at full retail price, or activate a device they currently own."

AT&T said the offer is "time-limited," but it's not clear when it expires. AT&T told us it's not revealing the end date "for competitive reasons." Some more details are available at the offer page. To receive both credits, customers need to pay their AT&T bill for at least 90 days. AT&T notes that there is an activation fee of $36 per line, a standard fee that apparently isn't waived for T-Mobile switchers. Each customer is limited to five trade-ins.

AT&T also recently announced $15-per-month discounts on data plans for customers with off-contract phones, an incentive that followed T-Mobile's move to eliminate service contracts.

T-Mobile is planning an announcement of its own on Wednesday at the Consumer Electronics Show. Rumors suggest that T-Mobile will be unveiling a switching incentive, but we don't have the details yet.

While T-Mobile came out of the failed AT&T takeover with new momentum, it may not be independent for long. Sprint is reportedly looking to buy T-Mobile and leave the US with just three major carriers. There could be a bidding war, as Dish is reportedly considering an offer.