I posted earlier in this forum but it has grown so big it looks like it's time for an update. We have the same intermittent (not 100% reproducible) issue as everyone else with outgoing calls. It is fixed 100% of the time by toggling the speaker phone on and off. The issue was first noticed the day after we got our phones. My phone works great and my girlfriend's phone is the one with the issue. We live together and make calls from the same location so that is not the problem.
After doing all of the resets Apple requested and getting a new SIM card for the first phone they replaced the first phone with a brand new one last Friday, Oct 21. It was a new phone because the 4S is so new that defective ones have not made it through the process of being refurbished yet. The first call on the new phone after walking out the the store had the same issue as the original phone. I was actually there almost 20 minutes after closing time because they could not figure out how to replace my phone that was bought online at the Applestore.com with a phone from a retail Apple store.
Currently, I am waiting for a call back from one of the advisors at Apple support. She is referring this forum posting to the engineers at Apple and asking for more information. She said it usually takes them between 1-4 days to respond to questions so I should know more information by Wednesday morning hopefully. I will let you know what I find out and if they have any insight to whether or not this is a hardware or software issue. Also, I would like to try the restore in DFU mode. The previous restores have been as a new phone, not from backup and did not fix the problem.
Thanks.