Getting sd card content

Hey there!im doing a last year school project using arduino and i have a goal to read an sd card and extract it's content to be used to send an sms using a gsm to a phone. i have made a txt in the sd card and i can read it in my monitor series like telephone=0739203020 however my teacher needs me to extract the info in such way that if i recieve telephone= my monitor series will show only the phone number and i'm lost and confused in what should i do or use

Have a look at the SD library.