Cisco recommends using the following guidelines when deploying call coverage using top-down, circular, and longest-idle algorithms:
•The Unified CM cluster supports a maximum of 15,000 hunt list devices.
•The hunt list devices may be a combination of 1500 hunt lists with 10 IP phones in each hunt list, or a combination of 750 hunt lists with 20 IP phones in each hunt list. However, an increase in a number of hunt lists can require increasing the dial plan initialization timer specified in the Unified CM service parameters. Cisco recommends setting the dial plan initialization timer to 600 seconds if 1500 hunt lists are configured.
Note When using the broadcast algorithm for call coverage, the number of hunt list devices is limited by the number of busy hour call attempts (BHCA). Note that a BHCA of 10 on a hunt pilot pointing to a hunt list or hunt group containing 10 phones and using the broadcast algorithm is equivalent to 10 phones with a BHCA of 10.
•Cisco recommends having a maximum of 35 directory numbers in a single line group configured to send the calls simultaneously to all DNs. Additionally, the number of broadcast line groups depends on the BHCC. If there are multiple broadcast line groups in a Unified CM system, the number of maximum directory numbers in a line group must be less than 35. The number of busy hour call attempts (BHCA) for all the broadcast line groups should not exceed 35 calls set up per second.
Busy Hour Call Attempts (BHCA) The BHCA is the average number of calls per hour per device during the busy hour. (For example, the busy hour for many systems is from 10:00 AM to 11:00 AM or from 2:00 PM to 3:0 PM.) Unified CMBE supports a maximum of 3,600 BHCA. When calculating your system usage, stay within the 3,600 BHCA maximum to avoid oversubscribing Unified CMBE. The BHCA consideration becomes significant when the usage for any phone is above 4 BHCA. A true BHCA value can be determined only by taking a baseline measurement of usage for the phone during the busy hour. Extra care is needed when estimating this usage without a baseline.
There are no limits that should be affecting the sizes you mentioned in your post.
Our standard setup is that if the receptionist is unable to take an incoming call, the call queues for 1 minute, if it is still not answered it is broadcast to all of the office (except management) - The broadcast group is around 86 phones.
All phones are SCCP
Do a search on the DN that is not ringing, check that there are no shared lines or ghost DNs
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