iax2 trunk - unable to create channel

Hello! I have 2 asterisk (13.3.0) servers and several iax2 trunks between them.
Sometimes IAX2 trunks sometimes become unavailable: i get following messages in console

WARNING[18384][C-0000082a]: app_dial.c:2431 dial_exec_full: Unable to create channel of type 'IAX2' (cause 20 - Subscriber absent)

On 1st server

iax2 show registry Host dnsmgr Username Perceived Refresh State 192.168.70.110:4569 N podolsk-sk 192.168.70.110:4569 60 Registered

ax2 show peers Name/Username Host Mask Port Status Description msk-holding-tru 192.168.70.110 (D) (null) 4569 (T) OK (18 ms)
on 2nd server (when this error occurs)

iax2 show registry Host dnsmgr Username Perceived Refresh State 172.17.20.141:4569 N msk-sktk-t 172.17.20.141:4569 60 Registered

iax2 show peers podolsk-sktk-tr (null) (D) (null) (null)(T) UNKNOWN

iax2 conf file looks like:

[general] bandwidth=high disallow=lpc10 ; Icky sound quality... Mr. Roboto. jitterbuffer=no forcejitterbuffer=no autokill=yes language=ru register => msk-sktk-trunk:passs@172.17.20.141 [podolsk-sktk-trunk] qualify=yes ; Make sure this peer is alive type=friend host=dynamic trunk=yes secret=passs context=from_msk deny=0.0.0.0/0.0.0.0 permit=172.17.20.141/255.255.255.255
This condition occurs once in 1 or 2 weeks, any ideas, how it can be fixed?