HI Nattapong...
Are you able to use the 'recall' button on your 'phone during an active call? If configured correctly AND supported by your VoIP service provider (*), this should place the active call on hold and play music to the other party while switching your end of the call back to the ATA so you can dial a third party or pick up a waiting call.
I believe the SPA-series have a firmware bug whereby the minimum & maximum times allowed for a hook-flash / recall signal were handled as minutes rather than seconds, so the default values of 0.1 and 0.9 map internally to 6 sec and 54 seconds! I found values of 0.005 (min) and 0.05 (max) (i.e. 0.3 sec and 3 sec) work fine, though the latter is really too high.
This bug was corrected in the ATA191, so if an SPA configuration which works is ported to an ATA device these values must be changed back.
The next possible issue is that the 'recall' button on telephones which conform to European standards (e.g. Uniden phones common in Australia) usually give a fixed-length pulse of about 100mS (0.1 sec) which is right at the bottom of the recommended range. This problem can usually be tested by operating the hang-up switch manually for longer: if that works but the 'recall' button doesn't, then I suggest using 0.05 sec (50mS) as the minimum.
This is rather a long-winded reply, but perhaps it will be useful to others too.
(*) Your ISP must allow more than one incoming call concurrently, since both the held party and the third party are handled by the ATA.
These vertical-service functions are really worth having IMO. Try ring-back which allows a user to camp-on an engaged party; the ATA polls that number and gives three short rings when they're free, then obligingly dials the number when the caller's handset is picked up.
