I'm confused about your description - you say four sites, but then in your description you have 1 HO plus four branches - that's five sites. If it's really five, you have a problem because the Multisite on a Room Kit only support four sites. The Room Kit Pro can do five - if that's your number you might want to go with that at the HO site. If one site is always going to be hosting these meetings, you don't need the Multisite option key on any other than that site. You seem to think you need one MS license for every call you want to make, that is not the case, the MS license enables the MS on the device, your number of calls then are limited by the capability of the unit.
I'm also not clear on the ISDN piece - are you going to put ISDN Links at all sites? One PRI is not going to give you very high quality calls to those other four sites - you should consider the internet as your WAN rather than ISDN. You could buy these endpoints at the MSRP price with subscriptions to register them to the Webex Cloud and use the cloud for your Multisite needs, you will have far better call quality and a far lower capital investment, granted with monthly fees going forward, but it's going to be a better solution that what you're suggesting, if I'm understanding you correctly.
Hope this helps - please clarify your ideas and I'll respond again.
