No..it's a standalone feature not integrated with other features..you can set time interval after which the call automatically goes on to voicemail or you can manually put it on voicemail while the phone rings by going into more options while the pho...
Yes, direct voicemail feature will be available..pls check https://r2.community.samsung.com/t5/Galaxy-S/Direct-Voicemail-feature-on-S23Ultra-OneUI-8-5/td-p/21409233?src=ShareByUserCMIt works, I'm currently using it on my S23Ultra
Most probably yes, if your S21FE is eligible for OneUI 8.5...however, some features may not come in final update unless Samsung decides to provide..expect the final update in 2nd quarter of this year..!!
