I know my phones have different settings called 'auto call' or something like that, basically you can set the options on the menu of the handset to either:
when someone rings and you want to answer
a) just pick up the phone and it answers automatically
b) pick up the phone and press the green button to answer the phone.
I always choose the latter because the display is small i cannot see who is ringing until I pick the phone off the hook and then I say "oh **** it's them calling me again" whoops they can hear me lol So I always have to chose the latter - I like to chose exactly when 'I' want to answer
Is this the problem, did you have to still press green to answer?