To offer an incoming X.25 call, the network must perform the following steps in sequence:
1) Channel selection - The physical channel/logical link to be used for the incoming call must be identified. The network may use customer profile information, network resources, etc., to choose the channel or the procedures of Step 2 below.
2) Physical channel/logical link establishment - If the physical B-channel or the logical link of the D-channel have not been determined by Step 1, the network may use the procedures in 6.2.2.3. The network may then proceed with Step 3.
3) X.25 virtual call establishment- The network establishes the virtual call using the procedures described in 6.3.
In the configuration for the ISDN virtual circuit bearer service, the choice of channel type to be used for the delivery of a new incoming call packet shall be made by the network as described below.
a) A new incoming call packet maybe indicated to the ISDN customer by a call offering procedure between the network and all user packet-mode terminals (see 3.2.3.2/X.31 and 3.2.3.3/X.31 [14]).
b) An incoming virtual call directed to a terminal with an established connection to the PH may be offered directly to the terminal over the established access connection without the use of Q.931 call offering procedures (see 3.2.3.1/X.31 and 3.2.3.2/X.31 [14]).