US Mobile Network Operators

http://en.wikipedia.org/wiki/List_of_mobile_network_operators

No. 1 Verizon Wireless 87.7M subscribers
No. 2 AT&T 81.6M subscribers
No. 3 Sprint Nextel 48.81M subscribers
No. 4 T-Mobile 33.5

Verizon, AT&T, and Sprint are all Tier-1 networks.
But T-Mobile? I only found 3 T-Mobile ASes in US.
AS10728 T-MOBILE-AS10728 - T-Mobile USA, Inc.
AS21928 T-MOBILE-AS21928 - T-Mobile USA, Inc.
AS22140 T-MOBILE-AS22140 - T-Mobile USA, Inc.
And based on this, AS21928 and AS22140 are stub networks with AS10728 not found.

Then how does T-Mobile build its backbone network in US? anyone knows?

Just found out AS21928 has 3 Tier-1 providers: Global Crossing, AT&T and Level-3.