Kalle had an valid comment on approval to hook things to the
phone lines.  I'm not 100% sure on this, but it seems to me
that if you build something yourself (such as CID) for yourself
and as long as you follow good engineering and construction
practices, you don't have a problem tying it to the phone line
(unless telco comes a knockin').  If you want a production item,
then approval is necessary.  Perhaps Andrew Warren can verify this
(I picked on him, because he seems to know quite a bit about alot
of things).  However, you can buy a pre-qualified module with the
telco interface.  Is it CerTek that has these things ?  They are
out there, somewhere around $20 or so.  Not sure about the name.