+--------+     +-----------+     +--------+
|1. snom | --> |2.  proxy  | --> |3. pstn |
+--------+     +-----------+     +--------+

- 1. sends an invite two proxy
- 2. sends a 401/407 back to to 1.
- 1. authenticates to 2.
- 2. consume_credentials()
- 2. route call to 3 while setting failure_route()
- 3. sends back 401/407
- 2. should uac_auth these

But instead from last point:

- 2. proxies the authentication request back to 1. instead of the 3.