Thank you for your help, Mango. Before I changed the Obi or router settings, I tried two troubleshooting things, and it appears that one of them resolved the problem for now. Hopefully, it will remain resolved.
I removed the CSipSimple app I had installed on my Android, that makes and receives Sip calls using the same Callcentric account as the Obi SP1. I don't use the app, and read online about how it can cause possible interference problems receiving calls.
I performed a full system reboot of the modem, router, Obi, phone, and computer. I brought up each one in proper order, then tested incoming calls. Things are back to normal again, and I have two-way audio on phone calls.
I am thinking that the problem might have been on Callcentric's end, or my internet provider, Time Warner Cable may have pushed an update that caused the problem. The system reboot may have taken care of it.
Speaking of Time Warner Cable, I do have a followup question, regarding your instruction in number 3), to try assigning a static IP to the Obi. Time Warner Cable assigns a dynamic IP, so in that case would I help or hurt my setup to assign a static IP to my Obi? I am not changing my settings now, since the problem is resolved, but I would like to know in case it comes up in the future.
Thanks for your help.