Extension dialing from auto attendant

E.164 is the format standard https://en.wikipedia.org/wiki/E.164

For extension dialing you do need the users to be enabled for phone system, have a proper number assigned to them with the extension (eg: tel:+14445556543;ext=6543) .... in this case your scenario should work no problem. If you wish to transfer one of the 10 options to an external number, then you don't need a user enabled for telephony, you can do it without that.

Thanks for confirming. I did see that it will be done based on digit pressed. As for the extension dialing, it makes sense what you said. I will look into this. When you say update the lineuri with a valid E164 phone number, is this a teams phone number?

Person calls a general number -> dials the extension -> call is forward to a teams number of the user -> then it's forwarded to the user's personal mobile phone.

let me rephrase the answer.....you can forward from the autoattendant based on digit pressed (0-9).....not extension. If you wish to use extension dialing you do need to enable the users for Enterprise Voice and update the lineuri with a valid E164 phone number. And......update the Azure AD with the proper lineuri including extension number

Yes you can forward to an external phone number from the auto attendant. You still need to setup some sort of pstn connectivity for Teams...either DR or Calling Plans...