Cannot Change Voice Mail Greeting

The no answer greeting fails to update. The T-Mobile process to effect a greeting change seems to work, but actually fails to work. This happened earlier this year and took 4 weeks to fix.  Well, temporarily fix. The issue is back, like Chucky. Happens with 2 phones, both iPhones, Pro 14, latest IOS upgrade. T-

Mobile technical support kept trying, failing and elevating to a “next” level of tech support. Finally, arrived at what was called “Level 4” tech support. The blame game on iPhone was bogus, never an IOS issue.

Was fixed and good for last 2 months. Now, problem is back. How do I go directly to Level 4 tech support to work problem again?