User Authentication in WCF Service

Question

Guys, i need a help.

I want to write a wcf service to send sms.

Befor sending sms i need to check user login details.

i wrote following methods separately

    [OperationContract]

    bool UserLogin(string username, string password);

    [OperationContract]

    bool SendSms(string messageBody, string tagname, string phoneNumber);

but i want to allow the user of  this service to login first and then use that sendSMS method.

How can i do that??