session-agent OOS timer

I'm looking at a call that failed to set up correctly... the last few messages are:

1)     18:19:17:482019          ACME --> SA          180 Ringing

2)     18:19:33:964655          ACME --> SA          200 OK

3)     18:19:33.983494          SA --> ACME          ACK

4)     18:19:34:469070          ACME --> SA          200 OK

5)     18:19:35:485815          SA --> ACME          ACK

This 200/ACK thing happens three more times until the SA starts sending BYEs... the first one is at 18:19:52:130740

6)     18:19:52:130740      SA --> ACME          BYE

This goes on for a few more rounds until eventually, the SA is placed OOS by the ACME

18:20:07.761 [SIP] SA x.x.x.x [xyz] PING TRANSACTION TIMEOUT to x.x.x.x

So, clearly the messaging is getting lost somewhere.. my question is: what timer does the Acme use to place the SA OOS?  I'm assuming it's waiting for 30something seconds after one of the 200 OKs was sent