Eugene's observations

When a phone or broadband fault takes longer than expected to resolve it is tempting to think a move to BT for phone or Internet Service provision would speed future problem resolution.  The assumption being that being members of the same Group, BT Openreach would give priority to BT Retail.  However, were this to happen an unfair commercial advantage would be enjoyed and so there are statutory rules in place to ensure this does not happen.

The Openreach policy that explicitly addresses the area of concern is at
http://www.openreach.co.uk/orpg/home/aboutus/equivalence/yourquestions/yourquestions.do#2

?2 Does BT Retail get priority return to service in the event of cable breakdowns or service interruptions?
No. Openreach must recover any cable breakdown or service interruption on a reasonable endeavours basis. This means that whatever can be restored first, must be, regardless of which CP [communications provider] is involved. We don't prioritise other BT lines of business in any recovery. To do so would be a breach of the Undertakings.?

?5 Does Openreach give any priority on Appointment Books?
No. Our appointment books are open for all CPs to book provision activities.?

The statutory rules are enshrined in Undertakings that BT gave to Ofcom as stipulated in the Enterprise Act of 2002.  The legal document that records this is viewable at
http://stakeholders.ofcom.org.uk/telecoms/policy/bt-undertakings/

These Undertakings establish an independent Equality of Access Office to oversee all aspects of the relationship between different arms of the BT Group.  They are also responsible for investigating complaints of non-compliance with the Undertakings. 

Complaints of malpractice (guidelines here) may be made by communications providers on receipt of a complaint by end users (you and me).  So if you have a grievance the first people to complain to are your phone or Internet service provider.  If you remain unsatisfied then you can request a ?deadlock letter? which enables you to progress your problem to resolution.  This is explained on the Ofcom website at
http://consumers.ofcom.org.uk/tell-us/telecoms/service-problems/

Because of these formal procedures it is hard to believe that BT would - even informally - be foolish enough to open themselves to litigation by the regulator and exposure in the press just to favour another arm of their Group.

It should also be noted that whichever ISP is chosen to andminister the line, the hardware will almost invariably be the same and the engineers who address problems would also be the same.  The difference between ISPs is just price and customer service - the technical aspects are unrelated to choise of provider.