Can't answer phone calls on BQ ubuntu edition

I noticed a recurrent bug lately on my BQ Aquaris Ubuntu edition. Sometimes, when I receive a call, the phone rings, but the window giving me the choice between taking or rejecting the call doesn't appear (with the red and green buttons)... So I have no way to answer the call, and the phone keeps ringing until the person calling eventually hangs up. very frustrating! I noticed also that when this hapens, the call does not even appear as a missed call in the notifications. I have to reboot the phone to get it back to normal. What causes this? is there a fix? How can I report this bug to the comunity?I am using the 21 version of the OS, and got the phone from bq a couple of weeks ago. many thanx ZTB