Please add OnePlus 6T to your visual voicemail compatible phones, you have a very limited range of device. Your support page says :
Google Pixel, Google Pixel XL, Nexus 5X, Nexus 6P, Google Pixel 2, Google Pixel 2 XL, LG Q Stylo+ and LG X Power 3 are the devices that support Visual Voicemail. More capable devices are also on the way.
In fact many samsung and oneplus devices natively support visual voicemail... can you listen your users and add some devices to your config file ?
Have you tried manually configuring the visual voicemail app? Unfortunately, Telus has chosen the worst implementations of their features. Who ever is responsible for that decision has severely hurt the company. WiFi Calling, HD Voice, Visual Voicemail, and so on, all use versions that have to be modified to work in Android and not the native versions contained in the OS. Google is partially to blame for allowing such stupidity as well.