Fetch value from redmine custom field

I am new to redmine.M using one api to send the messages(not workin)
My requirement is briefly given below.
I am using Redmine.I have a ruby code to send sms(from api).I have
written that code in redmine custom workflows.I have created one custom
field where user will type the mobile numbers manually.As soon as I
click on create button in redmine new issue page,the autogenerated
ticket id must be sent to the phone number which we type in tha custom
field.And this should occcur whenever the new issue is created.
Please tell me how do i fetch value entered in custom field and how to
pass that value in ruby scrpit??
The ruby code got from api has harcoded mobile numbers which I don
need.I want to pass those phone number values which i enter in custom
Please help me out.