A chat session in Kayako live support can be mailed, so to pick it up have it send to the address your whmcs reads.
+++ for the livesuport module in Kayko, it has many good sides, it is an out of the box solution, not much freedom to rearrange things, personally I prefer "desks" with more freedom in arranging the desk ("your workspace").
Making Kayako suitable for your needs is one thing - sure Kayako does what is has to if you just need a helpdesk. It is when bridging with other apps you might not get the results as expected.
Our favorite is the Cerberus Helpdesk, it is rock solid now and they have been listening to their community, it has the greatest advantage of them all, setup one helpdesk and generate code for all the sites you want to connect with the desk, you can generate forms and a helpdesk for the sites you wish to connect ... I think it is just a matter of time before it beats them all.... but here is also some work to do if you want it to be logged into other apps, many developers doesn't know the potential in Cerberus, despite the fact it is Open Source and it is not the simplest framework to understand for novices. Opposite the Kayako is Closed Source, and seperate licenses and installations will be necessary if you need their helpdesk on more sites.
For years now we have been waiting for a payment solution just to get prices delimited by a comma not a "dot", now Whmcs has it, I suppose their commitment is not only to the americans!
Anyway I could talk for days about such matters, it is up to you to decide...
Cheers
René