Some voice mails now go to T-Mobile Voicemail rather than Google Vocie

Years ago, I created a Google Voice account for the primary reason of directing Voice Mails to GV rather than (initially Sprint) T-Mobile Voice Mail. Until very recently, this has all worked brilliantly.

Now, it is mostly working, but the occasional Voice Mail is going to T-Mobile rather than GV. Most of these messages going there seem to be spam, though I still get some spam Voicemails on GV as well. I set this up (and I have verified that this is still the case) on my Galaxy S24+ by bringing up Dialer - going to Settings/Supplementary Services/Call Forwarding and for "Forward when busy", "Forward when unanswered" and "Forward when unreachable", I entered my GV phone number.

Any idea why some Voice Mails are going to the T-Mobile system?