Never buy anything over the phone. 'Blagging' is a common practice in call centres.
I used to always order over the phone, as I knew they had flexibility in discounts, and I preferred speaking to someone. I used to always record the call though, so there was a few different companies (Vodafone and Orange being two of them), where I'd sent a recording of the sales call in with the complaint.
I just don't think it's worth the hassle these days. I always sign in shop or online, then I instantly have the paperwork, to know exactly what I'm paying for.