Killing someone doesn't punish him, because he's dead and doesn't even remember that you did it, much less regret the things that motivated you to kill him.
Punishment is about more than instilling regret - it's also about extracting a price. I would certainly consider the prospect of being killed as "punishment," even though I wouldn't be subject to any suffering once it was over with.
Moreover, justice is about more than punishing the perpatrator. There's also repayment/satisfaction to the victims, deterrence of future offenders, and rehabilitation of the offender. Obviously, captial punishment doesn't play will with that last one, but it does frequently jibe well with the first two. The danger of the family of an executed criminal pursuing revenge for such must be balanced against the danger of the family of that criminal's victim pursuing revenge.
I can't believe that modern technology can't build a prison that blocks cell signals. Somebody just isn't trying.
I think the issue is more that guards and other non-inmate people at the prison need to have their cell phones work, for business and safety reasons. The inmates aren't supposed to have phones to begin with, so it becomes counterproductive to prevent the guards' phones from working because the guards aren't able to perfectly enforce that prohibition.
Studies show that the majority of men in U.S. prisons had violent, abusive fathers.
Studies also show that the majority of rape victims in the U.S. are male prison inmates:
http://www.hrw.org/legacy/reports/2001/prison/report1.html#_1_5