Where is the disconnect here?
[1] https://www.wired.com/2015/10/stingray-government-spy-tools-...
Stingrays (and presumably other IMSI catchers) don't actually act as a bridge to a legitimate network; they're a terminal connection, and effectively deny service to any devices connected to them. This is the default behavior, and it's how the FBI has claimed they're deployed domestically. The reason they don't cause major outages appears to be that they only accept connections from targeted phones; they could equally easily be used as mobile DoS beacons. And the reason they don't cause targets to abandon their 'broken' phone is that they don't have to run full-time for effective triangulation.
For Stingrays to collect calls, as outlined in your link, is effectively a whole new service on top of the basic behavior. The Stingray continues to be an endpoint for the targeted phone, but also extracts the GSM encryption key on the device. It then opens a new connection to a cell tower, and uses the IMSI and stolen encryption to impersonate the real device. From that point on, its a mitm attack.
So the only way a Stingray transfers calls is for targeted users, while configured in a mode that's supposedly not employed in the US. Harris is claiming they use a variant of this system to pass 911 calls, Wyden is pointing out that it's completely untested and doesn't even claim to support text-to-speech dialing.
(Source: Wikipedia is surprisingly comprehensive, primary source at: https://www.documentcloud.org/documents/1282642-07-08-29-200...)