In article <telecom24.191.5@telecom-digest.org>, PAT wrote:
> [TELECOM Digest Editor's Note: I would like to know one thing about
> prison phone systems, which are notoriously rip-off systems. The
> prison authorities _claim_ the recipients of phone calls from
> prisoners are not allowed to use _call transfer_ or _call forwarding_
> or _three way calling_ on calls from prisoners. I guess that is
> because the end result -- the person with whom the prisoner wound
> up conversing with -- would possibly not be on the 'approved' list
> at the prison. Does the prison phone system have the technical
> capability to restrict the called party's phone in that way? For
> example, I forward my phone somewhere, then you, in prison, call me
> as we agreed on. Or, you call me from the prison, I flash the
> hook and bring someone else on the line with me. The prison says
> in their literature that is impossible. Is it really? PAT]
It's certainly possible: the type of line is part of the calling party
identity in SS7 ISUP, and "prison" is one type of line (along with
normal stations, payphones, etc.) specifically to allow this sort of
service discrimination.
I used to have a reasonably good set of CLASS service documentation
but it is in storage. To know if call forwarding, 3-way calling, etc.
are specified to not work if one leg of the call is a prison line,
those Bellcore specs would be where to look. To know whether a given
switch implements it that way, you'd need the switch documentation;
maybe someone here with a documentation set for a recent 5ESS or DMS
generic can have a look. Of course, it may just say that the switch
does what the spec requires, in which case you would still need the
CLASS spec to know for sure.
Thor Lancelot Simon tls@rek.tjls.com
"The inconsistency is startling, though admittedly, if consistency is
to be abandoned or transcended, there is no problem." - Noam Chomsky
[TELECOM Digest Editor's Note: There was an interesting situation like
this in 1972, as ESS was first getting introduced around Chicago. The
first two offices cut over were Chicago-WABash in the downtown area
and Chicago-SUPerior on the near north side. Then the cutovers went
along until about 1980 or so when the final uncut exchange LOngbeach-1
in the EDGewater office went also. The oddity was in the way Call-
Forwarding was handled: -A- forwards his phone to -B- then later, -B-
forwarded his phone to -C-. A person dialing into B got forwarded to
C as you would expect, except a person dialing into A only got
forwarded to B, and not onward to C. Telco's rationale was that the
person calling into A 'expected to reach' B, not C, so that is why
the forwarding stopped short at that point, except if B dialed into A
and in that case it stopped with A anyway, regardless of A being
forwarded to B then B to C. PAT]
Thor Lancelot Simon (tls@panix.com)