Compare values between hosts and aggregated host

[markfree]

I have some real hosts (URA_X) with items that return the number of active calls. They are spread over 3 sites (group).
Each host has one such item:

Asterisk Calls - asterisk.active.calls - Zabbix agent item (user parameter)

I also created a virtual host (URA_Aggregated) with calculated items that return the aggregated sum and average of active calls for each site.

Average -- uras.calls.siteX[avg] - avg(last_foreach(/*/asterisk.active.calls?[group="SiteX"]))
Sum -- uras.calls.siteX[sum] - sum(last_foreach(/*/asterisk.active.calls?[group="SiteX"]))

This host also aggregates the sum and average of all sites.

Average -- last(//uras.calls.site1[avg]) + last(//uras.calls.site2[avg]) + last(//uras.calls.site3[avg])
Sum -- last(//uras.calls.site1[sum]) + last(//uras.calls.site2[sum]) + last(//uras.calls.site3[sum])

Now, I need to compare the number of active calls of a single host with the average number of calls of all hosts of a site.
In my mind, every single host should have a trigger that compares the last value of active calls with the (average number of calls of all hosts of the same site * 1.5).

At first I thought of a formula like this:

avg(/URA_X/asterisk.active.calls,#3)
>=
(last(/URA_Aggregated/uras.calls.siteX[avg],#1) * 1.5)

This is not supported because the trigger "belongs to templates with different linkages".

Any tips on how to do that?