Change Data-Type from Number to Text?

I'd like to modify the phone number data type (in open new ticket)

I changed the "Phone" text to "Room Number".

But our rooms doesn't only have numbers - like: 215

it can have text "E" subjects too - like: E43

Is it possible to change the datatype from number to text?

what do i have to do?

thank u very much.

… upload/include/class.ticket.php: change the array type from 'phone' to 'text'