A standard BT PSTN analogue line is required for ADSL. Certain BT services and/or configurations are NOT compatible with ADSL, these include:-
 
None BT Telephone Line 
Alarm Lines Multi Auxiliary Working Lines
Analogues Direct Dialling In (DDI)
BT Data Stream
BT Video Stream
Caller Redirect
Feature net
Internet Callers
ISDN2e / ISDN30e
Line concentrators
Lines served via TPON
Low Loss lines
Low User Scheme lines (only BT Retail)
Meter Pulse Facility
Night Busying
Out of Area lines
Payphone Lines
PBX Switchboard
Privacy Sets
Pulse Metered Line
Redcare Alarm Monitoring *
Surftime
Temporary Lines

Please note that this is a list of the most common PSTN Incompatible Products and is not
exhaustive.

How to check ADSL availability?

This can be done by using the Broadband Checker from within your 123vISP control panel.

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* See page 2, section 2.5.1 footnote at http://www.btwholesale.com/pages/downloads/broadband_community/ipstream_documentation
/btipstream_advancedservices_launch/btipstream_home_office_max_crf_v3.rtf