vn3 or basic-net3 would be the best choices for France.
Layer 1 issues should be related to the HW (box or cable); sounds like you have tested both. You might try to place the 1720 right at the switch with a shorter cable, to confirm if that makes a different. You might try another NT1, too.
You should check to see if the TEI's will be negotiated at power up or for each call; if the latter, then configure "isdn tei-neg first-call".
Finally, if you still have problems "debug isdn q921" (for TEI negotiation) and "debug isdn q931" (for call establishment) would be great to see. Keep in mind that you first need to get layer1 ACTIVE before you take this step.