Sipstation and most sip providers do not allow automated messaging. There are special things that need to be done with the carriers for these types of messages. You may be better off with a service like twillio for automated sms.
That said there is no call file. The messaging for sms in FreePBX for sipstation is not done through asterisk.
It is my understanding that anything NOT sent directly by a human. Can you get away with something like a vm notification? Probably. I am not sure what the threshold is per provider or carrier. Again as mentioned above I would just use a service meant for that like twillio. At $0.0075/message with no risk of having your number blocked by an upstream provider or cell carrier just makes sense… Side note I believe sipstation charges $0.01/message so twillio is cheaper too for this purpose.
With the discontinuance of Shared Short Codes and the rise of VoIP they have introduced 10DLC (10 Digit Long Codes) and have broken messaging into A2P (Application-to-Person) and P2P (Person-to-Person). The latter being how you do SMS/MMS on you mobile phone now and basically means a singular device to singular device(s).
VoIP numbers, on the other hand, automatically fall into A2P since it is pretty much done via applications in some manner with HTTP/S Restful APIs being the driving force. This means a couple of things:
First, Toll Free numbers where automatically classified as 10DLC before Shared Short Codes where terminated. These are handled differently and have higher charges from both your carrier and since 2020/2021-ish surcharges from other carriers (ATT/TMobile/USCellular/Verizon and even TextNow. Canadian carriers too.)
Second, standard local numbers are now classified as 10DLC and the carriers have created The Campaign Registry which is much like the RoboCall Mitigation Database. All businesses must register their Brand (business) with the Registry, they must create Campaigns (there are various types including Conversational) then they must associate their DID with a campaign. Brand registration requires a one time fee. There are recurring fees for campaigns and prices can vary based on volume and campaign type.
If your numbers are not associated with a registered campaign by March 1st 2022 (the final deadline after 3 push backs) then you will incur higher surcharges from ATT and TMobile/Sprint (right now) and those same carriers will treat your messages as P2P which means their spam filters will be applied along with any automated messaging blocking or filtering. Which means your messages will have a higher failure rate than success rate on their networks. This applies to all their MNVOs on their network too. So the destination number might not be directly ATT but if they are going over the ATT network all this applies.
We send SMS (to consented, opted in customers) using callfiles with Zipwhip and Twillio as our SMS service provider. We call a custom extension in asterisk ([Phantom-Call]), once the call is connected, the SMS is fired off via API in the dialplan script.
The reason we do this, as opposed to something more simple is that it is a blended campaign, text and voice.