Are you getting this error on hosted or trunking lines? I've seen this error using hosted lines behind a NetVanta router with the SIP ALG disabled and sip proxy set to transparent. The lines are operational, but this is normal and cannot be shut off unless you adjust your logging level.
There are sip parser errors and sip pre-parser errors. A sip parser error will show up in a:
debug sip stack message
output and print out the position and line that is causing the problem. Pre-parser errors are a bit more tricky. The sip debug above will not catch is so you will need to do a:
debug sip stack level errors
in order to get more information. Check the output from the sip stack level errors debug and if you have any questions, let us know.