you can use a PRI gateway like
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170507782933&rvr_id=129433344966&crlp=1_263602_263622&UA=%3F*I7&GUID=a6a5836412a0a0a9f4d5d9f5fe2c70b9&itemid=170507782933&ff4=263602_263622Or you can purchase a card for a gateway router.
Once you have it into your VoIP network you can purchase a multi line ATA like this one (
http://www.grandstream.com/products/gxw_series/gxw400x/gxw4024.htm) it supports 24 phone lines and would let you connect them all back to you legacy phone network. No new wires needed.