What you suggest can be done. The output of your existing voip ata will look like a PSTN line to the Line Port of the OBi110. However, there's a few things to consider:
Your existing ata coverts speech from digital to audio. If you now feed that audio to the OBi110, then it will convert the audio to digital, then back to audio to output to the Phone Port. All these conversions may lead to a lowering of speech quality, which may or may not be noticeable.
Assuming that you are in the US and you existing ata outputs CallerID in the standard North American format, then the OBi110 will add around four seconds delay while processing the CallerID before ringing its Phone Port.
QuoteMy idea is to connect that port to OBI 110 Line port and then use AA to screen calls by messaging telemarketers to hang up if they are selling or requite the caller to use an announced 2 digit number to continue the call.
To do precisely what you wish would require some modification to the Auto Attendant settings. However, the default setting Main Menu starts "press 1 to continue this call". This may be enough to stop telemarketers. After repeating the announcement three times the Auto Attendant will terminate the call if no response is received. Additionally, you would have to disable options 2 and 3 of the Main Menu – that's not too difficult if you only intend to use the Auto Attendant for screening telemarketing calls.
QuoteAdditionally I want to allow certain phone numbers to bypass AA, based on a small whitelist.
This is quite easy. If the white list is quite small than something like this would work:
Physical Interfaces > LINE Port > InboundCallRoute:
{(12222222222|13333333333|14444444444):ph},{aa}
The first rule is the white list sending calls from those numbers straight to the OBi110 Phone Port. The second rule sends all other calls to the Auto Attendant. The first rule could be made to look neater if there's a longer list of numbers by using a User Defined Digit Map.
If you want to try this out, then post back and myself or others will help with a more detailed reply.