All the wiring on the user's side of the NT1 is the user's responsibility. Your ISDN service supplier is unlikely to be able to help you with any problems you have here.

In the case of the BRI NT1, the standard (I.430) is very specific. You must use RJ45 plugs (although the standard does not express it in exactly these terms). These are most commonly found in structured wiring systems based on Category 5 twisted pair. This standardisation is very important because you can connect as many as eight devices to a BRI NT1.

If your NT1 is providing a PRI, you will have to use whatever cables are suitable for the sockets that are provided. Unfortunately, these are can vary according to the ISDN supplier. They might be two miniature co-axial sockets, RJ45 sockets, or even screw-down connections. The standard that covers this (I.431) allows some leeway. This not usually a major problem, since you are allowed to connect only one device to a PRI NT1.  

In the discussion below, all the distances cited assume that you are using Category 5 cabling. However, other types of cabling are also suitable such as AT&T PDS, Category 4, and Category 3.

In addition, these measurements apply only to the BRI. I.430 makes no mention of any distances at all - it only describes what the characteristics of the wiring should be. The lengths of the cables contribute to these characteristics. You may therefore see other values for distances quoted in other sources. Neither source is necessarily wrong, it just depends on the type of cable that you're using.