You've lumped together too many factors in your simple "ported-in vs. not ported-in" analysis. By itself, there is no reason whatsoever that there would be a difference. However, if you ported in a number within the last couple of days, it's been common to have problems receiving calls on the ported-in number, due to problems with the number porting system and the carriers who are using that system.
You need to troubleshoot this systematically. First, you need to determine if the inbound Google Voice number (regardless of whether you obtained it from Google's pool of free numbers, or you paid to port it in) is actually receiving inbound calls at the point of entry into GV's switch. To do this: go to Google Voice settings, and temporarily turn on "do not disturb". Call the GV number from some other number, not another GV number, nor from one of the linked/forwarding numbers on that account. For example, have a friend, family member or co-worker call it from their cell phone.
The expected behavior is that the caller will be sent directly to Google Voice voicemail. If that works, then the inbound GV number is provisioned properly and working properly. If the caller experiences some other behavior (busy signal, dead air, recorded error message), then there's a carrier problem.
Turn off do not disturb mode. Repeat the test call(s). The expected behavior is that all forwarding destinations should simultaneously ring, including all linked phone numbers, Hangouts and Chat/OBi. If not, you may have forgotten to add a check mark to the left of Chat in the old, "Legacy" Google Voice settings.
If the call(s) correctly reach the account's GV VM when DND is enabled, but not when DND is disabled, then the problem is with one of the linked/forwarding destinations. Inbound GV numbers can forward to up to six 10-digit US phone numbers, and/or to Google Hangouts, and/or to Google Chat (XMPP) clients. OBi devices act as Google Chat clients.