Spoiler: You can’t.
This one has been asked a thousand times on TechNet, and occasionally someone thinks they’ve found it, but they’re wrong.
Many companies want to generate lists of the numeric conference ID found in Skype for Business or Lync meeting invites for the dedicated or private meeting space. The trick is that although the alphanumeric conference ID can be found in the database (you can see this ID in the URL string generated in the meeting invite), the numerical/DTMF one isn’t stored anywhere. It’s calculated by an algorithm known only to Microsoft and mapped within the conference directory. Have I asked for it? Yes. Did I get it? No. I’ve been told by those within the Microsoft inner circle that the formula cannot be shared publicly, even under NDA.
The format of the actual number is as follows and documented here: https://technet.microsoft.com/en-us/library/gg398802%28v=ocs.15%29.aspx?f=255&MSPPError=-2147217396
<housekeeping digit (1 digit)><conference directory (usually 1-2 digits)><conference number (variable number of digits><check digit (1 digit)>
It is NOT the ConfID stored in the database (although this may well be part of the formula along with PSTN Local ID and PSTN Authority ID).
If someone manages to figure it out using some method, please let me know. I’ve used DBAnalyze a few times to try to pull it, but I haven’t received consistent results. My only thought would be a complex EWS scan of calendar directories for the information.