Two possible solutions:1) put in a small analog PBX. You can probably significantly reduce the number of sets, if each set can access all the lines. This also takes care of the issue of 'wire length'.
Additional benefits: "intercom" calling between phones, as well as 'conference' capabilities (internal and external), without losing audio level.2) Use a supplemental ring generator / ring extender. Viking Electronics is one of the better-known sources for such.
Option 1 _is_ more expensive, but you get a *lot* more for the money.[TELECOM Digest Editor's Note: Option 1 is exactly what I did here with my little 'TotalCom' PBX thing. And tiny little PBX/Line Sharing devices are not all that expensive these days. Two or three hundred dollars gets you a light-weight, all electronic device you mount on a wall somewhere and forget about it. Example PAT]