I want to change the telephone field from 'Telephone' to only 'Mobile Number' when a customer fills in their details
You can use locale files to change any labels.
Find the following files:
app/locale/en_US/Mage_Checkout.csv app/locale/en_US/Mage_Customer.csv app/locale/en_US/Mage_Persistent.csv
en_US is your current locale.
Find the following line
"Telephone","Telephone" and change the second part to your
If it is not working find all
"Telephone","Telephone" under folder
app/locale/en_US/ and change the second value.