Generally 1) Mitigate if possible from your service's end, while simultaneously 2) paging the dependent service's team to mitigate/resolve; and if this happens

Generally 1) Mitigate if possible from your service's end, while simultaneously 2) paging the dependent service's team to mitigate/resolve; and if this happens too frequently or if it was a particularly bad incident you can push the other team to 3) create a postmortem with follow up AIs if they already didn't do so.