T Mobile subscribers receive message "The number is not in service" when placing calls to active AT&T VOIP land line phone number

It's a routing issue to narrow it down you need to determine if it's all T-mobile numbers to all AT&T digital numbers or one T-mobile area code and exchange that can't get through. This is most likely a T-mobile issue.

It is all T-Mobile incoming calls to our 530 AT&T number.  Whether incoming from Washington state or California having different area codes 206, 707 or 530 any different area codes. All T-Mobile customers get the same response. “This number not in service” when calling to our 530 Area code

If that's the case it sounds like the port wasn't completed on the AT&T end. T-mobile should work with their counterparts in AT&T. If they won't your best bet is to file an FCC complaint. Have you reported it to AT&T as you described it in your last post?

One more question, are you sure this is unique to your number? This still sounds like a routing issue between T-Mobile and AT&T's digital switch that you're working out of, like T-Mobile is still routing it to the old switch prior to your switching to digital service.  

Have not yet reported this with AT&T.  Will report Monday 11/21/22.  Just learning this is a T Mobile / ATT VOIP issue.   All other carrier's mobile, wire land lines or VOIP lines are able to connect to our (530) number.  Changed over from ATT hardwire Land line to VOIP land line was just established 11/17/22.  We have had close to 100 phone calls from T Mobile customers getting the message "Line not in service".  FYI did not have this problem with ATT (WIRE land line) but ATT forcing switch by increasing wire land line cost to $300 P month not including long distance.  Other businesses in my area (REDDING, CA.) experiencing the same problem.  Unable to switch to VOIP without encountering the same issue.  Yes, I agree AT&T & T Mobile should work to resolve this issue.  Unfortunately for me and others, most likely, they won't do anything except say it sounds the other guy's problem.  

We are able to call out to T Mobile numbers.  They are just unable to place a call to our AT&T VOIP number.

FYI did not have this problem with ATT (WIRE land line)

That's because the routing would be different to the landline switch vs the digital switch since all that was done was switching from POTS to digital a routing issue is all that makes sense. It will be a T-Mobile routing issue but since you're not a T-Mobile customer it would be hard to report it to T-Mobile best bet would be for the T-Mobile customers to report that can't call AT&T digital service numbers. I have no idea how T-Mobile would accept this, that's why if you can't get any satisfaction from AT&T you'll be better served to file an FCC complaint.

Thanks for your insight.  Never filed an FCC complaint?????

FCC consumer complaints