The present day and age is the age of Gadgets and gizmos. And what if your mom has turned forty and you are planning for a surprise party for her. Since she has turned into a lady of forty so she might not be interested in electronics accessories or gadgets but surely a mobile phone is a must for your mother to be in touch with her. Even though she might have a mobile phone, still you can buy a new one for her and take away the old one from her. Do you have any more 40th birthday present ideas in your mind for your mom on her birthday this year? If not then the plan written above can be executed. 
The plan can be modified with a touch of feelings and emotions. That means your mother might hesitate to take the now mobile phone on her birthday and force you to yourself for your personal use. So what you can do is tell her that you have a surprise for her on her birthday and take a promise from her that you will tell her the surprise only if she says yes to accept that. Try convincing her by any means that the gift which you have bought for her is not costly. And say that it is a type of useful thing, equally useful as taking food. Hope you can convince her and give her the 40th birthday present.