I believe what you want is get an instance of the ContractDescription class. This class has a namespace property.

You can get an instance of this class using one of the GetContract methods. They have a Type parameter. So In your case, you could use this kind of call:

string myNamespace = ContractDescription.GetContract(
          typeof(IMyService),
          myPropertyInfo.DeclaringType).Namespace;

NOTE: you will also need the contract type (represented in this sample by typeof(IMyService))