Read last 5 Characters from a field

I am creating a Workflow class in c#.

I am looking to read in a field and pass its last 5 characters into a variable.

I can read the field in and do a check condition against it like:

querycontact.Criteria.AddCondition("phone", ConditionOperator.Equal, PhoneCheck);

I just want to read in the last 5 characters of the field "phone" into a variable something like this:

var number = ...("phone", 5) or something ... anyone know teh correct code I need to do this ?

Thanks.