I have been struggling with the same issue for a few weeks after my phone did a recent upgrade.
I have read many posts dating back through last year regarding this problem.
I might have stumbled onto a possible fix for this. (At least it is working for me)
My phone only does has drop calls/ no network connection and Mobile IP Registration Error messages when I don't have it on Sprint Only for Networking.
Under Settings/Wireless & Network settings / Mobile Networks
Network Mode - drop down
You are offered choices of: Sprint Only / Any CDMA / Any GSM/UMTS / Automatic.
I had probelms in all of those with the Mobile IP Resgistration error and dropped calls, except when I stay in Sprint Only mode.
I hope this helps.