is it working and supported that i use AP1240AG aps where both frequencys are configured for voice that the 7921 can roam between 2.4 and 5ghz?
for example i mount an AP inside a Hall 2.4ghz and 5ghz antennas outside the building for outdoor coverage?
I would suggest doing the opposite. 5 GHz for indoor and 2.4 for outdoor if 5 GHz outdoor is not feasible.
Yes the 7921 can roam across bands while sustaining a phone call using a single network profile. But the roam will not be seamless. First will have to exhaust the current BSS, then look to the other mode. If roaming to 5 GHz, may take a little longer vs 2.4 due to # of channels and 802.11h requirements for scanning DFS channels. So ideally would want to use a single band for seamless roaming. If wanting to use both bands, then need to set the user expectations that there may be a couple second gap when roaming.
The other topic is that the phone supports 5 different modes, where preference can be given to a particular band w/ or w/o fallback. If the phone falls back to the less desired band, then it will not keep monitoring the preferred band if associated to the less desired band and there is sufficient signal and low packet loss.
So to ensure the phones get to the correct band, ensure only that band is available in that area.