Buzzing/Hum on Landline

Last week I reported a fault with my landline using my mobile.  There is a buzzing noise when we make a call using the landline although the recepient cannot hear it but it is loud enough to abandon the call and use a mobile instead. During my call we went through all the usual things, unplug then reboot the Hub4, change your handset, still the same fault. After about an hour of this I was told I was being transferred to a member of the IT team who answered the call after about 20 minutes. Then repeated all the checks, this time with a few extras but all to no avail. As I was on my mobile I managed to play him the buzzing noise by holding it against my landline and he heard it and agreed there was an issue. He said there would be no point in sending an engineer out as the problem was not with my equipment. He told me I needed a software update and the fault would be fixed within 24-48 hours. I asked him who I should contact if I still had the fault after this period and he reassured me that I would not ned to, it would be sorted. Guess why I am writing this?  I know the landline is part of my package but it remains unusable and has been like this for a couple of weeks if not longer. The stand alone landline charge is over £20 so I really do not see why I should put up with this. Help!