I appreciate that the ISP is only one factor in the end-to-end Voip
'chain' however I presume it is a fairly important link in the chain
so my question is, from a voip pov, are there any ISPs to totally
avoid for SIP?

Do LLUs have any advantages/disadvantages?

Also are 'tunable' factors important such as contention ratios,
interleaving, traffic shaping options etc....

What isp's would you recommend/avoid?

Thanks,
Stefan