IAX vs SIP trunk

We have a set up as follows

SIPp <----->Asterisk svr #1<----> Asterisk svr #2

There is one SIP trunk between svr #1 and svr # 2.

And there is one IAX trunk between svr #1 and svr # 2

If SIPp dials 201, svr #1 uses IAX trunk and if SIPp dials 101, svr #1 uses SIP trunk. Both these calls terminate at SVR #2 with SVR #2 answering with “demo-thanks” message.

We have noticed that if the number of simultaneous calls is more than 5, SVR #1 is starts to get congestion when IAX trunk is used.

With SIP trunk, we do not have this congestion problem. I have run this test with SIP trunk for upto 10 simultaneous calls.

Can someone tell me if there is any additional config required for the IAX trunk?