Nearly half of the UK's ISP call centres offer incorrect advice about switching to and from full local loop unbundling (LLU) broadband providers.

Recent undercover customer research by Broadband Choices found that some customers were getting billed by both their old and new ISPs.

Issues surrounding the use of Migration Authorisation Codes also proved inconsistent, as did advice from regulator Ofcom.

"This is ironic as Migration Authorisation Codes were introduced by Ofcom to ease the issues surrounding migration," said Michael Phillips, product director at Broadband Choices.

"Clearly we cannot rely on ISPs to inform consumers on the best process to follow when switching."

Philips cited the case of a Mrs Farren, a TalkTalk customer who decided to move her broadband and telephone account from a Virgin Media ADSL connection to TalkTalk LLU.

To complete the process Mrs Farren obtained a Migration Authorisation Code from Virgin Media ADSL and the migration appeared to go smoothly.

However, she continued to receive bills from Virgin Media ADSL, as TalkTalk had not used the Migration Authorisation Code, despite taking the matter up with Ofcom.

"Ofcom has given us complete lack of clarity as to who is to blame in this matter," said Mrs Farren.

"While I appreciate that Ofcom cannot become embroiled in individual cases, I find it incredible that two large ISPs cannot agree who should take responsibility."

Phillips called on Ofcom to offer "clear advice on what action consumers should take if they find themselves in a similar situation".