It is Alameda County Social Services. It can be anyone employed by the agency (not all case workers). Might also just be the wrong number, I wouldn’t worry about it if no message is left.
510-670-8540 is a generic number, so when you call back you will not get the person that called you. Again if no message is left I wouldn’t worry about it.
Calls from this number are totally legit. This number is called a 'trunk line' it is assigned to Alameda County. Several county offices outgoing calls are routed through this number, it is not assigned to any one person or department and will not accept incoming calls. No reason to not answer calls from this number, it could be any of Alameda's county offices trying to reach you.
Got a missed call from this number to my cell. Haven't tried to call it back. With the above remarks about Alameda county services, wouldn't be surprised if it was unemployment office trying to schedule an appointment.