[Sip] SIP trapezoid (was Re: IETF solution for pairing cellular hosts)

Hello,

Is the SIP trapezoid needed in cellular networks?
This is a deployment question. The very simple logic
of my question is as follows:

For privacy reasons:
- cellular users don't publish their SIP URIs in a
  phonebook.
- unpredictable and long SIP URIs are recommended.

Thus the SIP URI will be:

       n-random-characters@domainname

which may be too long to read/type as we do
today when two cell phone users meet each other?

If so:
Any solution is welcome (e.g. pairing) to help
users exchange their SIP URIs easily. For example,
user names are locally published, I choose the name
of my friend in a wireless network, and get the SIP URI.

If such solution exists:
Why not use it also for exchanging a Mobile IPv6
home address. I.e., I get my friend's SIP URI and
home address concurrently. [bonus result: we can
establish IPsec since we already know our IP addresses].

Conclusion:
By the basic SIP privacy requirement we also came to
solving the routing problem.

What I am missing?

Thanks,
pars