True, with a rotary dial phones, you could simply place a short across the two pulse leads (blue and green, or blue and blue) to disable the dialing capability.
With touch tone phones, we would either get ones that do not have a polarity guard in the dial, or disable it. Once that was done, we would then intentionally reverse the tip and ring of the lines we didn't want them to dial out on. The polarity could then be left correct on the lines or intercom where dialing was desired.
