Timer to control telephone , Plz Help !!

[ahmed osama]

Hello all

now i need any tips or circuits for th following

I need a timer , so after the user dial his number , the timer should begins and if the calling is more than 5 min ,it should close the conversation but the incoming calls should be opens

i need any info about that (circuits , books , sites , EVEN a commerical products..etc)


thanks all

bye

 

[electronicus]

Look fere: **broken link removed**
Your can modify the schematic, for the phone.