sip vs iax2 trunk

I'm having some trunking problems and I would like some input with this. I have 2 Trixboxes connected via full T1.
Trixbox A 192.168.1.1
Trixbox B 192.168.2.1

Trixbox A has 2 full pri connected via sangoma card. I have had problems off and on with s-congestion and channel unavailable errors on calls from B to A. Below is my trunking configuration

Trixbox B

IAX2 Trunk

Peer Details:
host=192.168.1.1
username=USER
secret=xxxxxx
type=peer
trunk=yes
qualify=yes
context=from-internal
disallow=all
allow=gsm

User Context: USER
User Details:
host=192.168.1.1
secret=xxxxxx
type=xxxx
trunk=yes
qualify=yes
context=from-internal
disallow=all
allow=gsm

SIP Trunk
Peer Details:
insecure=very
qualify=yes
host=192.168.1.1
username=USERS
secret=xxxxxx
type=peer
trunk=yes
context=from-internal
;nat=yes
disallow=all
allow=gsm

User Context: USERS
User Details:
insecure=very
qualify=yes
host=192.168.1.1
secret=xxxxxx
type=user
trunk=yes
context=from-internal
;nat=yes
disallow=all
allow=gsm

Trixbox A

IAX 2 Trunk

Peer Details:
host=192.168.2.1
username=USER
secret=xxxxxx
type=peer
trunk=yes
qualify=yes
context=from-internal
disallow=all
allow=gsm

User Context: USER
User Details:
host=192.168.2.1
secret=xxxxxx
type=xxxx
trunk=yes
qualify=yes
context=from-internal
disallow=all
allow=gsm

SIP Trunk

Peer Details:
insecure=very
qualify=yes
host=192.168.2.1
username=USERS
secret=xxxxxx
type=peer
trunk=yes
context=from-internal
;nat=yes
disallow=all
allow=gsm

User Context: USERS
User Details:
insecure=very
qualify=yes
host=192.168.2.1
secret=xxxxxx
type=user
trunk=yes
context=from-internal
;nat=yes
disallow=all
allow=gsm

I have created 1 SIP trunk and 1 IAX2 trunk between the boxes. The SIP trunk has terrible audio quality. I have tried different codecs, nat = yes and nat = no. The calls almost always go through via sip but the call quality is terrible. Over IAX2 the calls go out about 80% of the time and quality is good. Any suggestion would be greatly appreciated.

Thanks
Rusty