PJSIP: configuring outbound registration

My ITSP published a very technical specification that states:

The UE shall not add the IMPU in the user part of the SIP URI that is used as the Contact Address. This would reveal the IMPU to the called party even in case OIR is applied.
The full specification is available here: https://www.telefonica.de/file/public/1158/SIP-Interfaces.pdf?attachment=1

Could someone with a deeper understanding please explain in a less technical fashion, what exactly this means for correctly setting up my outbout registrations in pjsip.conf, especially in regard to the configured ‘Contact’ header field? From my (limited) understanding, I am NOT supposed to put my DID number in the user portion of the ‘Contact’ header?

I’m unfamiliar with IMS (which is what this is referring to) so not sure what an IMPU is in relation to the Contact header, so you may be correct.

It would seem so. This is what wikipedia has to say:[QUOTE]
The IP Multimedia Public Identity (IMPU) is used by any user for requesting communications to other users (e.g. this might be included on a business card). There can be multiple IMPU per IMPI. The IMPU can also be shared with another phone, so that both can be reached with the same identity (for example, a single phone-number for an entire family).[/QUOTE]
But that still leaves the question what I shall put into the user portion of the registration’s contact header. From my understanding this is what the ITSP’s server will use in the INVITE when a call comes in for me, and it’s also the extension where the call will enter asterisk’s dialplan. So in a way it appears logical to put the corresponding DID number there. If I do not configure anything, my understanding is that asterisk will use the ‘s’ extension. This would mean I would have to use a separate dialplan context for each of my four DID numbers, each with it’s own ‘s’ extension.