Ask what kind of DSLAM they use and then ask if it is the best DMT type available.
DSLAM = DSL Access Multiplexer
DMT = discrete multitone ( over 250 x ~ 4kHz channels which each can support 50kbps )
POTS = plain old telephone system
If you dont have a splitter with the high pass & low pass splitter the audio will load the ADSLS signals and you may be able to hear the ADSL dialing a connection on the lowest channel ( 8 Khz or so. )
So Yes you need a POTS splitter if not included in Modem. Otherwise you get interference, so adding a splitter might recuperate the loss.