This is the debug output from the 3640 when it tries to establish the connection, which doesn't work:

01:33:17: BR2/0 DDR: Attempting to dial 21
01:33:17: ISDN BR2/0: TX -> SETUP pd = 8  callref = 0x1F
01:33:17:         Bearer Capability i = 0x8890
01:33:17:         Channel ID i = 0x83
01:33:17:         Called Party Number i = 0x80, '21', Plan:Unknown, Type:Unknown
01:33:17: ISDN BR2/0: RX <- CALL_PROC pd = 8  callref = 0x9F
01:33:17:         Channel ID i = 0x89
01:33:42: ISDN BR2/0: RX <- DISCONNECT pd = 8  callref = 0x9F
01:33:42:         Cause i = 0x8190 - Normal call clearing
01:33:42: BRI2/0: wait for isdn carrier timeout, call id=0x8020
01:33:42: ISDN BR2/0: TX -> RELEASE pd = 8  callref = 0x1F
01:33:42:         Cause i = 0x8090 - Normal call clearing
01:33:42: ISDN BR2/0: RX <- RELEASE_COMP pd = 8  callref = 0x9F

ISDN status after the attempt starts, but before it gives up:

Global ISDN Switchtype = basic-net3
ISDN BRI2/0 interface
        dsl 16, interface ISDN Switchtype = basic-net3
    Layer 1 Status:
        ACTIVE
    Layer 2 Status:
        TEI = 64, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
    Layer 3 Status:
        0 Active Layer 3 Call(s)
        CCB:callid=8023, sapi=0, ces=1, B-chan=1, calltype=DATA, hdlctype=HDLC-TRUNK
    Active dsl 16 CCBs = 1
    The Free Channel Mask:  0x80000002
    Number of L2 Discards = 0, L2 Session ID = 0

There is a call control block, but no active call. The output when there is no call attempt at all in progress is different, as it's like this:

Layer 3 Status:
        0 Active Layer 3 Call(s)
    Active dsl 16 CCBs = 0

Which is again different from when a connection has successfully been made:

 Layer 3 Status:
        1 Active Layer 3 Call(s)
        CCB:callid=C, sapi=0, ces=1, B-chan=1, calltype=DATA, hdlctype=HDLC-TRUNK