Iax2 Trunk A > B ok but B >A drops after about 1 min

running IAX2 trunk between 2 servers
calls from server A to B are fine but calls from B to A get disconnected after about 1 min

Server A
[2User]
type=user
trunk=yes
secret=2Secret
host=192.168.20.4
disallow=all
context=from-internal
allow=g729

[Server1]
username=1User
type=peer
trunk=yes
secret=1Secret
qualify=yes
nat=yes
host=192.168.10.3
disallow=all
context=from-internal
allow=g729

pastebin.ca/495221

Server B

[1User]
type=user
trunk=yes
secret=1Secret
host=192.168.10.3
disallow=all
context=from-internal
allow=g729

[Server2]
username=2User
type=peer
trunk=yes
secret=2Secret
qualify=yes
host=192.168.20.4
disallow=all
context=from-internal
allow=g729

pastebin.ca/495218