I've done some work on this. XLN Telecom purchased OneBill Telecom:
http://www.comms-dealer.com/industry-news/xln-telecom-successfully-completes-mig...The 0844 567 6777 customer services number for OneBill is the same IVR (menu) as XLN's 0844 880 9000 number. The options are:
1. Broadband enquiries
2. Existing telecoms enquiries
3. Enquires from non-customers
The same IVR is also on 020 7793 5530 /5531 and 7793 5983 and freephone numbers 0808 178 7875 and 0800 917 2739. I have listed these accordingly.
OneBill Telecom used to be part of Comms Factory, which is (or was - not sure on this) owner of Yourcalls.net. Some of OneBill Telecom's unverified alternatives were 01903 ones, similar to those of YourCalls.net, so I think that these are now longer any use and I have removed them. They were all 01903 70340x and 41x.
All the entries for XLN Telecom have now been named as such, so do a search by name and they will all come up.