Skype-in Telephone Numbers no longer accept Toll Free SIP "From" or P-Asserted-Identity - sip

Question

FACTS
For the past seven years, I've made outbound calls via SIP. Calls have a SIP From header with a standard (non-toll) telephone number, and a P-Asserted-Identity header (PAI) with a name and a toll-free number. The caller ID display at the destination shows the name and toll-free number (if supported by the phone and service provider, of course).
In recent days this scheme has stopped working for Skype-in numbers.
DISCUSSION
Starting approximately 2018-01-22, I've seen a jump in calls rejected with SIP 408 messages.
My SIP termination provider says that most of the calls were rejected upstream of Level 3. Our internal checks of the problem lead us to believe that the rejected calls are for the most part to Skype-in telephone numbers.
I've tried multiple tests with various formats of the PAI and the From. Using the abbreviations TFN for "toll-free number," here's a list of what I've tried:
From with non-TFN, PAI with sip: TFN and name (which has worked for seven years)
From with TFN and name, PAI with sip: TFN and name
From with non-TFN and name, PAI with sip: TFN and name
From with non-TFN and name, PAI with sip: TFN and no name
From TFN with name, no PAI
From non-TFN and name, tel: PAI TFN with no name
From TFN and name; tel: PAI TFN with name
and a few more besides. Regards of the format of the PAI (TFN, non-TFN, name or not, tel: or sip: format) and regardless of the format of the From (with or without name, TFN or non-TFN) -- if either From or PAI contain a toll-free number, the call is rejected with a SIP 408 or an occasional SIP 403.
My SIP termination provider does not know how to solve the problem. Questions from my SIP termination provider to upstream carriers have not yielded a solution.
I speculate that Skype is deliberately blocking TFNs from calling Skype-in numbers for reasons I don't understand. At present (2018-01-31) Skype-in will cheerfully display any non-TFN number I send in the PAI without checking to see if I own it or not, which implies that this not a general security measure (e.g., an anti-swatting measure).
ACTION
My questions are:
Is this a well-known, recent problem?
What is the solution to terminating at Skype-in numbers while sending a name and a toll-free number?

Answer 1

The solution to this problem turns out to be quite straightforward: somewhere past my SIP provider and past Level 3, there's a deliberate block of the specific toll-free number ("non-toll DID") we have been using.
At a colleague's suggestion, I tried different toll-free numbers under our control in the PAI, and they all worked. As a sanity check, and since PAI's are never (in my experience) checked for validity, I found I was able to successfully call using invalid numbers, e.g., 1 800 111 2222.
I therefore conclude that the problem is the specific number we're sending in the PAI. I'm in touch with my SIP provider; now the hunt is on to find out who is blocking that number.
EDIT
After a conversation with Skype, our number did start working again... but while I did get "This is an issue were [sic] working to fix," I was never able to find out the scope of the problem: our number specifically, groups of numbers, and which Skype-in numbers were blocked.
Regardless, Skype support, reached via answers.microsoft.com, did resolve the problem.