iPhone 6S Visual Voicemail Delayed or Lost While on Wifi Calling

My wife does not get cell service at work, but she is able to make and receive calls on the hospital WiFi via WiFi Calling.

However, if someone calls her and leaves a message, she will not receive the voicemail through Visual Voice Mail. Sometimes they come through after a long delay, but usually she gets no notification whatsoever if the message is left while on Wifi Calling (even after regaining cell signal). If she calls in manually to her voicemail box the messages are always there.

For example, last Thursday she received a voicemail while she was at work and didn't get a notification on her phone until Sunday.

Does VVM not work via WiFi Calling? Or is there something wrong with her phone or TMobile settings that could be causing this?