Snort needs a couple of variables for SIP rules
after the update of the rules of this night, Snort didn't restart successfully (curious because I didn't make any change to the config).
However after a bit of investigating, I found out that the problem was in Snort_Voip.Rules, where are defined the variables $SIP_SERVERS and $SIP_PORTS that I cannot set anywhere in the service interface (Snort 2.9.1 pkg v. 2.0).
The only workaround was to set in "Advanced configuration pass through" the following lines:
Var SIP_SERVERS $HOME_NET
Var SIP_PORTS [5060:5090,16384:32768]
then Snort started.
This two variables should be added to the "Define servers" page of the package, the values above are the known default for both options.