Fetch value from Redmine Custom Custom field

I have created custom field in redmine wher we type the client phone number manually.Also written a ruby code in redmine custom workflow to send sms to client when ticket id is raised.In the code I can type phone number but i actually want to fetch that phone number which we type before creating the new issue..So how do i fetch the value of phone number from the custom field and link it to the code? Please suggest me