T-Mobile: T-Mobile has a similar story to tell as Verizon. Instead of allowing you to set up a specific basic voicemail, it instead directs you to Apple to set up Visual Voicemail. I dont see anything that you have written when I try to set up voicemail on my new Iphone11. I tap on voicemail and the only option is 'Call voicemail' … then it doesn't accept my mobile number.
2. Access the dial pad. You'll need to bring up the Phone app's dial pad, as if you were going to call an actual person. To do this, tap the dial pad icon near the bottom of the screen.

A good voicemail greeting is short and professional, lets people know that you’ll get back to them, and invites callers to continue engaging with a call-to-action. You should also show your personality if you’re in an industry or role that allows that. If your industry is more conservative, however, you’ll want to keep humor and personal touches to a minimum. A greeting Your name Your company A simple explanation for missing the call (e.g. you’re away from the phone or are on holiday) A rough estimate of when you’ll get back to the person An alternative person to reach out to (if you’re out of office) An alternative mode of communication (if you prefer email or text) A call-to-action such as “Leave a message” or “Send me an email at [email protected]”
If you’ve already created a custom voicemail on your iPhone, you can tap Play to see how your voicemail sounds. If you’re interested in creating a new message, tap the Custom button. From here, you can start recording your message by tapping Record and tapping Stop whenever you’re finished. Once you’re finished recording, tap on Play to make sure the message comes across as you’d like and hit Save (in the upper right) to save it as your voicemail.