calls not going to voicemail

I just switched my cell phone service from AT&T to T-Mobile and bought a new XS iPhone. I just learned that missed calls aren't going to my voicemail. My husband called, and it rang repeatedly without ever getting my outgoing message. The same thing happened when I tried calling his phone, so I'm guessing something got lost in the switch. Our outgoing messages are still there, so I'm not sure how to fix it. Any suggestions would be greatly appreciated! :-)