Welcome to our Member Community!
We are investigating a very unusual bug (that’s not related to the voicemail issue described on our status page) where a handful of members are experiencing what you’re describing - calls in progress that suddenly drop into voicemail.
This is not something our Community would be able to solve for you. I’d like you to open a Help Ticket, provide a description like you did here, and if at all possible, provide some recent examples of calls that were dropped. If at all possible, please include the phone number that made the call, the phone number that received the call, and the time and date of the call (with time zone.)
Please ask that the issue be investigated as part of Master Ticket # 2104587.
Please let me know if you need help getting that Help Ticket opened up.
(Pinging @Ambassadors for awareness about this Master Ticket.)