mISDN channels in down state

Hello folks, from some days i’ve got some mISDN channels which remains in down state…
If i use show channels, i’ve got this output

mISDN/5-u8606 (None) Down (None)
mISDN/4-u7750 (None) Down (None)
mISDN/4-u7000 (None) Down (None)
mISDN/4-u6980 (None) Down (None)
mISDN/5-u6321 (None) Down (None)
mISDN/5-u1625 (None) Down (None)
mISDN/4-u387 (None) Down (None)

But Asterisk are working very well though…
Any suggestions?
Many 10x!