Last year, Sprint started its “cut your bill in half” promotion, even though customers’ actual savings turned out to be more like 20% once customers acquired a new phone for Sprint. Today, Sprint announced that they’re continuing the deal and expanding it to T-Mobile customers. But not all T-Mobile customers.

As is the case with all promotions that sound really great, you need to read the fine print. If you don’t pay attention, you could get stuck with a plan that doesn’t work, or an enthusiastic salesperson might switch you to something that doesn’t work with the plan you’re switching from or how you use your phone.

For example, here’s the fine print that applies to all carriers:

Activ. Fee: $36/line. Plans: Offer ends 01/07/2016. Savings through 01/08/2018. Req valid port from AT&T, Verizon or T-Mobile active wireless line to consumer account. Plan includes unlimited domestic calling and texting and unlimited int’l texting. Select add’l int’l svcs are included. See sprint.com/globalroaming. Max of 10 phone/tablet/MBB lines and one data share group per account. At least one phone req. Subsidized devices incur an add’l $25/mo charge. Plans exclude unlimited music and video streaming, data carryover, tethering and cloud options that other carrier plans may offer.

Your bill may be cut in half, but if you’re switching from a carrier like AT&T, which offers tethering to customers who haven’t accepted a device subsidy, or from T-Mobile, which exempts certain bandwidth-gobbling streaming services from data caps, Sprint won’t match those.

What’s the catch for T-Mobile customers, who are part of the promotion for the first time? Customers who have an unlimited plan from Big Magenta aren’t eligible, even though the plan that they’d be switching to at Sprint is a similar unlimited plan where users get switched to slower 2G speeds after using up their data allotment.

Or, when you blow the fine print up to a more legible size:

Discount offer limited to T-Mobile’s advertised Simple Choice rate plan prices as of 11/16/15 for 2GB, 6GB and 10GB for non-discounted handsets only; tablet and MBB rate plans excluded. T-Mobile unlimited data rate plan excluded. Data is not shared among multiple lines. Add’l on-network high-speed data allowance may be purchased at $15/GB.

Wait, so customers need to buy extra data allowances if they’re coming from T-Mobile? What about the other carriers? People switching from Verizon or AT&T have to pay overage fees of 1.5 cents per megabyte, or $15 per gigabyte, if they go over their high-speed allotment. That may mean that the T-Mobile version is harsher, charging customers for the entire gigabyte as soon as they go over.

While Sprint is promoting the deal as allowing customers to try the network out for a month, be cautious: check with neighbors and co-workers to find out whether Sprint actually works in the areas where you travel often. One family who tried to take advantage of this same promotion last year ended up stuck with a four-figure bill for Sprint service that they decided not to keep.

Sprint to halve bills for those who switch from rival networks [Reuters]

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