The only solution I can think of is to calculate it yourself using the alert data and flow data. Might be difficult to do since the request may be incrementing the source ports, so using a 5-tuple wouldn’t match up. I can use Brim to calculate it. This may not be a solution for you though. There also maybe something you can do with Lua scripting and create the output logs you’d need. Otherwise you may have to infer the amount of packets. Since only 1 will trigger every 100 attempts in 60 seconds, you know there’s at least that number but it could be higher.
For each INVITE request, does the source port change?