Final answer:
In the English language, when a pronoun is an object, 'her' is used instead of 'she'. Hence, the sentence should read 'We can phone my grandma and ask her'.
Step-by-step explanation:
The correct pronoun to use would be 'her'. Therefore, the sentence would be 'We can phone my grandma and ask her'. In English, when a pronoun is an object, as in this case, we use 'her' instead of 'she'. 'She' would be utilized when the pronoun is a subject. Here's another example for clarity: 'She likes ice cream' (as a subject) but 'I gave the ice cream to her' (as an object).
Learn more about pronoun usage