I hate to be the bearer of bad news but Cisco Wi-Fi phones don't play nicely with Meraki wireless. We had 7921s, 7925s and 8821s when we started moving to Meraki a few years ago from old Cisco WLCs and 1242s. We had random frequent one way audio on all of them initially and in the last year we finally got 7925s to behave unless you let them go to sleep when they won't ring... Last week we installed the latest firmware on our 8821s and the one way audio, though still occasionally happening is definitely much better. We have been on 25.13 since it was released and I believe this is what got the 7925s (sort of) working. We tried 26.2 in production as it seemed okay at our HQ but it was a big fail in the venues with regular occasions where suddenly dozens of clients (iPads and Cisco Wi-Fi phones) would lose connectivity and not get it back unless we rebooted the AP. We currently are running 25.14 in the venues but are yet to know how stable that is. If you haven't already tried, then Cisco Wi-Fi phones definitely like to be on lightly loaded radios (no more than 10 clients) and prefer 5GHz, though I'm not sure this applies to the 7921 as I can't remember if it had it! We have only the phones, Wi-Fi security radios and our iPads on the 5GHz radios and this means they seldom go over 10 clients. The rest of the users have to all squeeze on the 2.4GHz radio, but I've seen that working with 100+ phones/laptops!
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