Complicit but not convinced the networks are to blame
I'm no fan of the networks (they can do their own excuse making) but from a dispassionate read around this subject two things stand out:
1. The business, which was already holding significant senior debt, was loaded with £200m of junior debt in autumn 2013 which was paid out immediately to the Private Equity owners as a special dividend. I suspect this was secured on the business with no recourse to the owners so i) the interest rate would have been [relatively] punishing and ii) the PE boys could just skip away without penalty when it all went kaboom. If there was a viable business worth investing in the £200m would have been invested in it, or not borrowed at all to keep the cost base as low as possible. The business was bought by the current owners for £700 but had almost that in debt when it went into Administration.
2. The networks claim P4U wanted a margin twice that demanded by Carphone, citing interest payments on debt loaded on the business as the reason. There may be some variance between high street retailers and allowing a slightly bigger margin may have been desirable so the networks weren't left with just Carphone on the high street but it isn't for the networks to significantly increase their cost base as a consequence of the funding/ownership structure of P4U.
The networks may have a smoking gun in their hands, but the gun was loaded, passed to them, and the trigger pulled by the Private Equity boys (who I bet have handsomely enriched themselves from this whole affair while happily blaming their suppliers for the whole mess).