Comment by GauntletWizard

This is important to stress. SRE often takes first-line oncall, but "We" support the service. On a well behaved service, having SRE support is an extra debugging-focused engineer when things go wrong. But that engineer is rarely called up, and can support the common parts of failure (network issues, hardware failures, the things that are out of any one team's service team's control) for multiple teams.