I would of thought that ISDN would be the perfect solution and if not settle for a standrad POTS line and use one of the folloing;
 
http://www.tieline.com/

http://www.glensound.co.uk/GS-MPI004%20Broadcasters%20Mobile%20Phone.htm
 
http://www.pots.audiotx.com/
 
http://www.sonifex.co.uk/codecs/index.shtml
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GIT/MU/U dpu s: a--> C++>$ U+> L++> B-> P+> E?> W+++>$ N K W++ O M++>$ V-
PS+++ PE++ Y+ PGP t 5 X+ R- tv+ b+> DI D+++ G+ e(+++++) h--(++) r++ z++
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