Calls from Samsung Galaxy S7 Edge aren't going to voicemail

When I call friends who have iPhones (all on AT&T, same as I am) and they don't answer, their voicemail is not picking up. I called 3 separate ones to test it and got the same results. I called from my office desk phone and their voicemail did pick up. I called a friend with a Samsung and her VM picked up.

These same people are able to call and leave me VM.

Any ideas? This has gone on for a few days. Thanks!!!

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