Don't forget that lots of times people with verizon can reply to your voicemail by pressing 8 and you will not get a call indicator. Make sure that you friends are not just responding to your calls.

That said, when was the last time the phone was rebooted? Sometimes things just hang. Also, is your phone checking email every 30 minutes or is outlook "pushing" mail to your phone every 30 minutes. If it is push then that should not be causing the problem since it should only send you messages with new email to your outlook account.

My phone checks my internet email accounts every 30 minutes and I have no problems such as you described other than when my phone is locking up and needs to be rebooted. Sometimes if a call comes in while the phone is fetching email, it will cancel the email sync and the call still comes thru.