I have missed a load of posts, so I might be spewing my usual waffle of ignorant tripe here, but I asked many years ago about having internet ONLY phone line, or just an incoming line, to help lower the bills, and they told me ( I am sure it was BT at the time ) but they said that you have to have outgoing call capabilities as connecting to the internert requires the Modem to dial out, even Broadband needs to dial ZERO.
I assume that not that much will have changed regarding that point.