Well, built-in backdoors would be problematic for the same reason admin panel backdoors or Clipper-chip backdoors would be problematic.

But if the Congress end up extending 'reasonable expectation of privacy' to cover any "common carrier" (which I think is long overdue) I'd be very surprised to see them leave out any ability to surveil, wiretap, etc., at least from the major telecoms. That doesn't need to mean backdoors or warrantless snooping though.

Nobody (except the ACLU, if you phrase the question carefully) will say that court-authorized Title III wiretaps violate the 4A. The questions involve the process, standards, procedures, and scope. Telcos + CALEA == a lost cause. Not so with going up the protocol stack.