Is there any issue using the phone line for the lift cars for an ADSL connection? A strata building wants an ADSL connection for their security system interrogation.

There is only one line servicing 2 lift cars. I imagine we would just need to fit a filter at the MDF and take off for the ADSL. If they wanted to use another phone as well in the office, could or does the lift car phone have the ability to 'takeover' the line if needed by someone in the lift?
Cheers
Pilbara