There are a lot of reasons why a depositary bank may not be able to verify funds; e.g. the drawee bank is closed, the drawee bank does not verify funds, or the teller at the depositary bank did not have time to make the call. All are equally unrelated to a reasonable belief that the item will not be paid.
When you call the drawee bank and they indicate that "funds are currently not available" you have reasonable cause to doubt collectibility. When the drawee bank calls you and says the item is being returned for "insufficient funds" you have reasonable cause to doubt collectibility. If you have a reasonable belief that the drawer or even the drawee bank is insolvent, you have reasonable cause to doubt collectibility.
However, when the drawee bank does not answer the phone or, as a matter of policy, does not confirm funds, then you don't have an ounce of additional information and you do not have reasonable cause to doubt collectibility. There is no presumption that only funds from checks where funds can be verified are subject to Regulation CC's availability requirements.
The reason is bogus and should draw criticism, but not everyone agrees.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.