Starlink rival AST SpaceMobile says its satellite-based phone technology will work outdoors and indoors once the system is up and running for AT&T and Verizon customers. 

In a letter to partners and shareholders, Texas-based AST SpaceMobile argues that it's "uniquely positioned" to supply satellite-based cell coverage across 100% of the continental US. 

The company signed deals with AT&T and Verizon to use the carriers' 850MHz radio spectrum, which “offers superior signal penetration in the low band cellular range," AST SpaceMobile says. 

“Low-band frequencies travel longer distances and penetrate deeper into buildings and through foliage compared to higher frequencies,” the company added. “This ensures our signals will reach you wherever you are, even in remote areas with limited terrestrial infrastructure.”

AST SpaceMobile didn’t elaborate on the expected indoor performance. But the company’s past tests with its prototype BlueWalker 3 satellite achieved a 14Mbps download speed and powered video calls while outdoors. 

AST SpaceMobile will face competition from SpaceX's cellular Starlink system. In March, SpaceX said its technology can beam cell phone signals indoors and outdoors to various smartphone models, with download rates hitting 17Mbps. 

The competition between both companies promises to help mobile carriers serve users in cellular dead zones, particularly in rural and remote areas. But a key question is which service will offer better performance. In the meantime, Monday’s letter from AST SpaceMobile claims the company will offer "groundbreaking" coverage for AT&T and Verizon customers. 

However, AST SpaceMobile still needs to launch its first production satellites into space, which might not happen until Q3 or Q4. And it'll require dozens more to offer continuous coverage over the US. SpaceX is already operating about 90 Starlink satellites for its own cell system with the aim of launching the service later this year with T-Mobile, pending FCC approval.