Glip Calling using assigned Site Number

I might be missing something but for the life of me, I am unable to call out using the Site Number. I can call out using the full account number but when I have users setup with Multi-Site and want users from site Boise for example to use a Boise assigned site number, and users from Tampa to use a Tampa assigned site number it won't let me. All I see is the general account main number which I do not want to use. Am I crazy or am I missing something?