Call routing procedure on outgoing calls

User	dials	correct call route	correct user	mapping, number in use	Destination line
VoIP telephone	14	None	VoIP softphone	14	Internal
VoIP telephone	11	# (Standard)		#: 11	ISDN
ISDN telephone	14	PBX	VoIP softphone	14	Internal
VoIP telephone	0 555 555	0#		00241#: 0 555 555	ISDN
ISDN telephone	0 555 555	PBX		555 555	ISDN outside line
VoIP telephone	0 0123 666 666	00#		0#: 0123 666 666	SIP provider

Internal call between two VoIP terminal devices.
Internal call from VoIP to ISDN. In the first pass (without the standard routes), the number '11' does not match any of the routes. Similarly, no matching entry can be found in the list of registered users. In the second pass, the standard route meets '#' (entry in the call routing table depicted above) and directs the call to the ISDN line unchanged. The PBX receives the call on its internal ISDN bus, recognizes the called number as an internal MSN and again forwards the call to the internal ISDN bus to which the respective ISDN terminal device is connected.
Internal call from ISDN to VoIP. The ISDN PBX recognizes the destination number '14' as an internal MSN and directs the call to the corresponding internal ISDN bus. The Call Router receives the call to '14', does not find a matching entry in the call routing table but does find an entry in the list of registered users.
External call from the VoIP into the local telephone network. The entry in the call routing table depicted above matches the number dialed. The Call Router completes the area code for the local telephone network and sends the call out to the ISDN line. Only now does the SIP PBX removes the '0' outside-line access prefix and completes the call to '0241 555 555' via the ISDN outside line.
External call from ISDN into the local telephone network. The ISDN PBX recognizes the destination number as an external destination, removes the '0' outside-line access prefix and completes the call to '555 555' via the ISDN outside line.
External call from VoIP into the national telephone network. The entry fits in the call routing table here. The call router removes the '0' prefix for access to the outside line and completes the call to '0123 555 555' via the SIP line. If the SIP line is not available, then the call is made over the ISDN line. In this case, the '0' is not removed from the destination number in order to gain access to an outside line through the PBX.