Friend, in your function you have tried to use the field phonecall_ids of the user witch the id = 72.

You can check it in you database.

Or you can try to print res.partner(72,).phonecall_ids when you call the function and you will find False.

So, try to:

-Eliminate the call to this result in your function.

Or,

-Fill the field phonecall_ids to your user.

Regards.