As Derek said lines should be around +/- 48VDC on hook. Under load (off hook) Typically under +/- 10V DC and Ring 90-110VAC
Quick dirty by all means not a comprehensive troubleshooting of DSL.
1, Get an extension cord and plug the modem in to the NID (
http://www.dslreports.com/faq/16236) Using a phone cord under 6ft in length or with the manufacturer provided cable. Some folks really like the 100ft dollar store cords, if you find this trash it, move the modem.
If the modem works here then the service is good. If it doesn't work here call the telco.
2. Make sure every answering machine, phone, medical alert device, house arrest ankle tracker box, water meter box, alarm system has a filter. Some techs will install a hole house filter in the NID or next to the NID ( a similar looking grey box, but smaller, with a wire to/from the NID) With these in place only one jack will work.
3. Make sure if there is 2 ports on the filter that all devices are in the "phone" side. Make sure the modem is not filtered.
4. If all filtering is proper and you can't get it to work on any existing jack run your own cable from the NID (cat3 or better)