SAR filings are required by both FinCEN and regulatory agency regulations. Both regulations agree that you must make supporting documentation available to "appropriate law enforcement agencies" on request. Neither regulation countenances the possibility that your bank would have phone conversations regarding the supporting documentation. For several reasons, I would not.
Telephone discussions of events that take place after a SAR is filed are completely off the reservation. A regulator should not ask. You should not tell.
The FDIC and its counterparts are not law enforcement agencies nor are they supposed to be acting as shills for law enforcement agencies in conducting investigations. It would take a touch of brass to ask you to review the circumstances surrounding a previous SAR and see if you felt filing an update was appropriate (wink, wink). However, expecting to be kept abreast of recent developments via telephone conversations is way, way out of bounds.
Even if a bank can verify that it is a real law enforcement agency such as the IRS/CID or the FBI on the phone, the waiver of the subpoena requirement for supporting documentation is pretty significant. Yet, no one intended that obtaining information from a bank should reflect the same degree of difficulty as ordering a pizza. If they ask you for documentation regarding SARs you have already filed, ask them to show up with identification and an expectation they will be required to sign an itemized receipt. If they want anything else, tell them you need a subpoena.