grammar - past tenses

hello,

I would be grateful if you answered my questions:

1)This a sentence form my grammar book ,
I was washing my hair when the telephone rang.
If the above is corect why
I was watching tv when my sister talked on the phone.
is incorrect.

2)What is the difference between the sentences below?
a) I was watching TV while my sister was talking on the phone.
b)I watched TV while my sister talked on the phone.

thanks

[Tdol]

1- phone rang = short action my sister on the phone = longer action

2 Both are fine.