https://www.cisco.com/c/dam/en/us/td/docs/telepresence/infrastructure/tms/admin_guide/Cisco- TMS-Admin-Guide-15-6-1.pdf Number Allocation When routing a conference, Cisco TMS allocates a random number in the assigned ranges to the conference in the following manner: 1. Cisco TMS tries to find an unused number or alias on a bridge within a 4 hour window around the conference (4 hours before the start time and 4 hours after the end time) for both new and an edited conference. 2. Cisco TMS checks for 4 hour window around the conference irrespective of the Extend Conference Mode settings. 3. If there are no unused numbers or aliases during that time frame, Cisco TMS tries a 2 hour window, then 1 hour, 45 minutes, 30 minutes, 15 minutes, and finally settles on a number or alias that is unique for the exact duration of the conference. This is so that back-to-back meetings are not allocated with the same number or alias unless no other number is available, so participants are not at risk of dialing into the previous conference. For recurrent bookings, Cisco TMS uses the same number or alias for all occurrences. When a single instance of a recurrent meeting is edited, this single exception occurrence can get a different dial-in number. This also applies to bridges behind TelePresence Conductor if variable alias patterns are used.

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab11/collab11/presence.ht ml