When I put a phone in auto-a for his 802.11-modus, the phone will go to the b/g band, when available, if it goes out of range for the a-band.
And will switch to the a-band if it's in range again.
But, 'while making a call', if the phone comes in range of a AP with the a-band available, will it drop the call and switch to the a-band, or will it stay on the b/g as long as possible.
The auto-a means it will use 802.11a if available at power on. If not will use 802.11b/g. If travel out of 802.11a range and 802.11b/g is available then will use the less preferred mode. If you travel back into 802.11a coverage, it doesn't mean the phone will revert back to 802.11a if 802.11b/g signal is still present.
We encourage not to roam between bands.
But yes you can roam and sustain a call across bands with a minimal interruption.
This is explained further in the 7921G Deployment Guide (see page 50).