A budget Windows Phone phablet for AT&T and a slew of smaller handsets could be on tap for Mobile World Congress, according to one in-app advertising firm.

AdDuplex, a company that serves cross-promotional ads on Windows 8 and Windows Phone apps, says it's seen a lot of unidentified device activity on its network in the run-up to MWC. Judging from the devices' screen sizes and resolutions (see slide 15 in the link), it's not hard to guess what kinds of phones Microsoft has up its sleeve.

For instance, one device has 5.7-inch 720p display, and has been spotted running on AT&T's U.S. network. While that's on the large side for a phone, the pixel density would be much lower than AT&T's existing Lumia 1520, suggesting a cheaper oversized handset.

AdDuplex has also discovered a couple of 4-inch phones with 480-by-800 resolution displays (including, strangely, one with the same model as last year's Android-based Nokia XL) and a couple of 5-inch 720p handsets. The smaller phones seem likely to target the low-end market, while the larger ones could be more in the mid-range.

In any case, AdDuplex hasn't seen any signs of a flagship phone. There hasn't been any indication of new Windows Phones from HTC, Samsung, or LG, either. With Mobile World Congress kicking off on March 2, we'll get the full scoop soon enough.

Why this matters: Microsoft has made no secret of its plan to establish Windows Phone in low-end and emerging markets, and that's likely what the MWC lineup will be focused on. While the lack of a killer high-end phone is disappointing, Windows Phone is still too short on reasons for people to consider it over an iPhone or high-end Android handset. Windows 10 could change all that with deeper hooks into your PC, but it looks like Microsoft is holding out on the hardware front until the software is ready later this year. That might not be such a bad idea, given Microsoft's occasional difficulties with getting wireless carriers to deliver timely updates.