how to use additional phone number for simultaneous use of zoom meeting and receive calls

If your Zoom Phone number is not ringing your desktop client while you are in a Meeting, check your settings on the Phone tab. You may have disabled it. Check to ensure you have not checked "Hide incoming calls while in a Zoom meeting"

Hello!  Trying to hit each of your points here
For the first, as mentioned by Rob, be sure that you do not have that box checked.  Incoming calls should prompt you and you'll have the option to decline, accept, or merge into your meeting.

For your second question, each user can have up to 10 DIDs, and they'll all behave the same way, either toll or toll free.
If you are on your PC on a meeting and use a desk phone, you can absolutely remain in the meeting with audio and be on your desk phone.

For your third question, a toll free number costs the same as a toll number, as you mentioned.  The difference is you do also pay the inbound toll free per minute rates to go with that.

Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.

Rick