This article describes the issue of the T1 WAN link connection randomly dropping.

 The T1 link randomly drops;  but it does recover.

For troubleshooting WAN link issues, look at the T1 interface statistics, which can be obtained from the command- request support info . 

Then check if you see Yellow alarms .  With JUNOS, if there is one yellow alarm, it will detect a failure and will bring the WAN link down.  These are more sensitive than other WAN link devices. 

For example, let's say the WAN link interface output is as follows:

Physical interface: t1-4/0/0, Enabled, Physical link is Up
  Interface index: 131, SNMP ifIndex: 138, Generation: 134
  Description: Sprint MLPPP Sequence 3
  Link-level type: Multilink-PPP, MTU: 1510, Clocking: External, Speed: 1536kbps, Loopback: None, FCS: 16, Framing: ESF
  Device flags   : Present Running
  Interface flags: Point-To-Point SNMP-Traps Internal: 0x4000
  Link flags     : None
  Hold-times     : Up 0 ms, Down 0 ms
  Keepalive settings: Interval 10 seconds, Up-count 1, Down-count 3
  Keepalive statistics:
    Input : 2251 (last seen 00:00:07 ago)
    Output: 2261 (last sent 00:00:02 ago)
  LCP state: Opened
  CHAP state: Closed
  PAP state: Closed
  CoS queues     : 8 supported, 8 maximum usable queues
  Last flapped   : 2010-04-12 07:05:25 CDT (06:18:45 ago)
  Statistics last cleared: Never
  Traffic statistics:
   Input  bytes  :            867362110              1101840 bps
   Output bytes  :            352686303               280832 bps
   Input  packets:              2457842                  167 pps
   Output packets:              2437438                  290 pps
  Input errors:
    Errors: 0, Drops: 0, Framing errors: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, 
L2 mismatch timeouts: 0, HS link CRC errors: 0, SRAM errors: 0, Resource errors: 0
  Output errors:
    Carrier transitions: 3, Errors: 0, Drops: 0, Aged packets: 0, MTU errors: 0, Resource errors: 0
  Egress queues: 8 supported, 4 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0 best-effort              2432918              2432918                    0
    1 expedited-fo                   0                    0                    0
    2 assured-forw                   0                    0                    0
    3 network-cont                4520                 4520                    0
  DS1   alarms   : None
  DS1   defects  : None
  T1  media:            Seconds        Count  State
    SEF                          0            0  OK
    BEE                          0            0  OK
    AIS                          0            0  OK
    LOF                          0            0  OK
    LOS                          0            0  OK
    YELLOW                      19            1  OK
    CRC Major                    0            0  OK
    CRC Minor                    0            0  OK
    BPV                          0            0
    EXZ                          1            1
    LCV                          1            1
    PCV                          0            0
    CS                           0            0
    CRC                          0            0
    LES                          1
    ES                           0
    SES                          0
    SEFS                         0
    BES                          0
    UAS                         27

The workaround is to define a holddown timer for the T1 link. The recommended command is:

set interfaces t1-4/0/0 hold-down up 0 down 100

The above command example is assuming that your T1 card is on slot 4 of a J-Series router.  This holddown timer will prevent link flap, if just a single yellow alarm is encountered.