I had Infinity installed on the 1st November. It is very slow and goes off every time I pick up my phone. I contacted BT Help and received an e-mail back saying contact a number. It would cost me £1.53 a minute. That's not my idea of HELP!
It sounds like your extentions were installed using the "star system" of wiring. You need a telephone engineer (not Infinity one) out to reconfigure it.
You should not be charged for this.
If you have problems arranging this use the link in my sig to contact the mods here. LInclude a link to this thread on the form.