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Losing sync when the phone ring means that either the IW (inside wiring) is marginal (time for a NID test), and if it fails there, then the Filter/Splitter at the CO is bad/marginal .(cross-connects filter) However, if the inside wiring is found bad, then it is time for a "Home Run" installation.
There is another possibility as shown in this FAQ:
»DSL FAQ » My DSL disconnects when I answer the phone
Q: My DSL disconnects when I answer the phone (#326)
A: This is a symptom of bad internal wiring, or problems with filters, splitter or sockets.
Another possibility is an MTU on the line. An MTU, is a Maintenance Test Unit. In days long gone, it was used to remotely check for problems on your phoneline. It seems these lovely units that consist of an Op-Amp and a bunch or resistors, capacitors, etc ... cause major problems with DSL. These are generally located in your phone closet or basement (where the phoneline comes from the street into your house/apartment).
You can have your phone company do a line check and they should be able to determine if you have one on your line and approximately where it is located.
(Thanks to Doctor Olds)