I tried editing an existing number with pounds and pauses. When I removed the pound keys, Siri was able to dial the number immediately. When I added them back, Siri just froze, as if waiting for something to happen, for about a minute.
I also tried editing an existing number with no pauses or pounds. I added a pause, and it still dialed. I added a pound at the end, and Siri froze, waiting again. Then I moved the pound to the middle, and it still froze. I removed the pounds altogether, and Siri was able to dial the number again immediately.
This could be a critical function for some people, since many voicemail systems require the caller to press pound to move through their systems. Manually pressing pound adds a great deal of time and inconvenience, to say the least.
EDIT: The dialing with pound numbers worked until roughly a week ago, wen I upgraded to iOS 15.4(19E241) on an iPhone 11