I'm out of options, please helpt me with a suggestion.
My deskphone stops ringing when my status is set to away and option 'ring mobile en deskphone at te same time' enabled.
I would say this is normal behaviour, "ring my extension and mobile at the same time" is under "Available" section, and you said that your status is set to "Away", so incoming calls will be routed by "Away" setup
you can check server activity log and see what is happening when incoming call came to the 3cx, why is your mobile is not ringing, do you have configured outbound rule for mobile number?