a conventional isdn network loop is depicted in part in fig1 as comprising an isdn local switch ( ls ) 11 which receives transmissions from within the network and forwards specified messages via digital subscriber line 12 to the designated network terminator ( nt ) 13 associated with the addressee isdn customer . the nt 13 located on the customer premises converts between digital subscriber line and isdn passive bus signal formats . a number of isdn terminals , such as dual bus terminal 20 of the present invention , may be connected to isdn bus 14 by means of four - conductor cable via digital signal port 15 . dual bus terminal 20 includes basic components of a conventional isdn terminal , namely q . 931 message controlled switch ( qcs ) 21 , including s - interface transceiver and b - and d - channel multiplexer / demultiplexer elements , analog / digital ( pcm ) signal conversion codec ( cdc ) 22 , and call control ( cc ) component 23 . a call received from a distant location via ls 11 is normally passed to a conventional isdn terminal through nt 13 and is communicated from bus 14 via isdn signal port 15 of the terminal to qcs component 21 . after demultiplexing at qcs 21 , conversation is forwarded , under the direction of cc 23 , to cdc 22 from which they are output as analog signals on cable 16 - 17 to conventional electro / audio analog signal transducer handset 19 via analog signal port 18 . two - way isdn network communications will normally follow this processing path in a common isdn terminal and there is thus no opportunity for a second local premises terminal to participate in a conversation . a dual bus terminal 20 of the present invention additionally comprises a coupling component ( cpl ) 24 situated on the analog side of cdc 22 , for example between cable connector sections 16 , 17 . by means of cpl 24 a communication tap for analog conversation signals is provided between line 16 - 17 and two - wire line 25 leading to a second analog signal port 26 . an ac - coupling network , as shown in fig2 provides an effective means for safely appending two - wire line 25 to the four - wire analog line 16 - 17 of a conventional isdn terminal even in the presence of an external dc power source . an extended two - wire communication line , such as may be obtained by isolating conventional on - premises analog phone wiring from the subscriber &# 39 ; s loop , provides an analog bus 27 to which analog port 26 is connected . other dual bus terminals , such as dbt 20 &# 39 ;, may likewise be connected to that bus . analog bus 27 may also be connected by wire pair 28 to a conventional analog phone 29 which may then be employed in this simple configuration as a means for conversing with a distant caller once communication has been established with the isdn network by means of dbt 20 . in operation , dual bus terminal 20 may be used in the usual manner to establish communication with a distant terminal in the isdn network , yet by means of analog bus 27 additional terminals are enabled to participate in the ensuing conversation . the basic configuration depicted in fig1 provides a constant such extension phone capability . it may , however , be desired in some circumstances , e . g . in the interest of privacy , to temporarily disable this capability . as one of a number of options which may be included in an embodiment 30 ( fig3 ) of the dual bus terminal , a user - operable switch 36 provides a means for selectively limiting periods of extension line operability . such a capability is one which is not presently available in conventional analog phone systems . when the premises extension phone network comprises analog phones 29 the necessary 48 vdc power may readily be supplied from a source 34 in each dbt 30 . in anticipation of such multiple power sources , element 34 preferably includes resistors sufficient to provide protection against inadvertent wiring errors during installation . alternatively , the power may be provided from a single dbt 30 or from an external source , not shown . since the one isdn terminal , such as dbt 20 , initially involved in a call setup is designated by isdn protocol to be the controlling terminal , any discontinuation of the communication at dbt 20 would normally likewise discontinue the communication between all extension terminals and the remote terminal . it may well be , however , that it is desired that any terminal on the premises be allowed to continue a communication after the controlling terminal user has completed all intended business . an optional detector component 38 in a controlling dbt 30 senses continued extension phone activity and provides such flexibility either independently or at the discretion of the user of dbt 30 . such a detector might comprise a current flow sensor , e . g . a common ferrod device or a simple relay 31 , which is activated by the level of current flow during the off - hook condition of any of the extension terminals . where the extension terminal is another dbt , appropriate resistors in its switchhook element 32 provide a signature similar to the off - hook impedance of an analog phone . in the simpler mode noted above , this sensor initiates a signal , such as led 33 , by which the user of dbt 30 is made aware of any off - hook extension terminal . the user of dbt 30 may then allow continued extension use by remaining off - hook or otherwise maintaining the connection between cpl 24 and bus 27 at switchhook element 32 . visible signal 33 also provides an additional benefit of alerting one to an inadvertent reversal of wiring between analog bus 27 and a second powered dbt during installation . for independent continuation of extension phone communication after an on - hook condition at a controlling dbt , a similar sensing circuit may be employed , for example utilizing an led 35 and photodetector 37 , to provide a logic signal to call control ( cc ) component 23 via line 39 to inhibit the initiation of the clearing of the active isdn call as long as any extension phone is in use . also the logic signal is used to control means , not shown , for maintaining the closure of switch element 32 even after a physical on - hook condition at a dbt , except when that terminal is in the &# 34 ; null &# 34 ; state . higher levels of call control may also be permitted for extension terminals in the present invention . for example , a degree of control over cc 23 of dbt 30 may be provided to an analog terminal 29 on bus 27 such that upon the occurrence of an off - hook condition in terminal 29 while the dbt is in the &# 34 ; call received &# 34 ; state the logic signal generated at photodetector 37 of the dbt detector 38 initiates the sending of a connect message from dbt 30 . in the earlier - noted manner the closure of switch element 32 is effected by use of this logic signal . an analog terminal 29 could thus be used to directly answer an incoming call . in such an operation , however , it is preferable to allow a sufficient time delay after the analog terminal off - hook condition to enable a user at an isdn terminal to take direct control of the call . for further variations of the present system , additional analog buses may be included in the premises network and may be maintained independently for different groups of extension terminals or may be selectively ac - coupled with other buses or connected with various terminals , all at the discretion of the user . additional options made possible by the dual bus arrangement of the present invention should readily become apparent to the skilled artisan . these and other embodiments of the invention are nonetheless to be included within the scope of the invention as set out in the appended claims .