IAX2 trunk not working same time with IAX2 extension

I use Elastix2.0x create IAX2 trunk connected two PBXs are working well.
A PBX SIP extension can call B, also B can call A.
But after add IAX2 extension in A or B, they can not call both, and then I move IAX2 extension they are working,and both can call.
A PBX trunk: iax
context=from-internal
host=192.168.1.251
secret=123456
trunk=yes
type=friend
qualify=yes
requirecalltoken=no
qualifyfreqok=60000
qualifyfreqnotok=10000

B PBX trunk: iax
context=from-internal
host=192.168.1.254
secret=123456
trunk=yes
type=friend
qualify=yes
requirecalltoken=no
qualifyfreqok=60000
qualifyfreqnotok=10000