2 asterisk pbxs - no sound and variable congestion

I have 2 asteriskNow pbx’s - one in the UK, one in India. Basically, I get very patchy results between the two - sometimes a call work, sometimes it doesn’t and never with sound.
When a call can not be made I receive

[quote][Feb 25 21:59:48] NOTICE[2099]: chan_iax2.c:2931 __auto_congest: Auto-congesting call due to slow response
– IAX2/astnow2-4 is circuit-busy
– Hungup ‘IAX2/astnow2-4’
== Everyone is busy/congested at this time (1:0/1/0)
== Auto fallthrough, channel ‘OSS/dsp’ status is ‘CONGESTION’[/quote]

Between the two is a VPN with no - natting. I’ve tried these settings to reduce the congestion issue

bandwidth=low disallow=lpc10 jitterbuffer=yes forcejitterbuffer=no autokill=yes qualify=5000 maxjitterbuffer=500 tos=lowdelay trunktimestamps=yes

The thing I’m not too sure about is that all other traffic on this VPN works, http, samba, ssh. Is IAX a lot more picky about needing a certain level of connection before it makes a call?