I had set this back to 5060 and this seems to help. However, I'm getting a 488 'not acceptable here' error from the SBC. I took a look at the SIP ladder, and indeed the 3300 is not specifying any codecs. Here's the relevant section in the 183 message:
v=0
o=- 2049 2049 IN IP4 [redacted IP]
s=-
c=IN IP4 [redacted IP]
t=0 0
m=audio 0 RTP/SAVP 18
In a working SIP exchange, the host responds with proposed codecs. Not the case here.