I performed some additional tests tonight. I have two Callcentric/Telengy inbound DIDs, both with the same area code and prefix, and differing only by one digit. I wasn't using the second number for anything other than a fax mailbox, so I temporarily added and verified it on my GV account. Inbound calls to the GV number immediately rang that CC DID. Humph. So I deleted and re-added the original CC DID, tested again, and both numbers had the delayed ring this time.
I also have two local (425) ported-in DIDs hosted by Callcentric, and both of those ring immediately when added as GV forwarding numbers. So, this is definitely another classic Bandwidth--->transit carrier--->Telengy issue.
I think the problem is intermittent and dependent on which route the calls take between GV and Telengy...similar to some of the other GV-->CC problems over the years, as I mentioned above.
There's nothing that can be done about this, other than a) waiting it out to see if it mysteriously gets resolved, or b) give up on it and forward to Google Chat instead, or c) get (or port in) a local, non-Telengy DID from Callcentric, and pay about $60/year for unlimited inbound service. PhonePower has a similar price, but apparently for inbound + outbound + E911, so likely a better deal if you make a lot of call.