I have a journey with entry of salesforce data and an SMS activity which should contain data coming from service cloud. I have already added the field in entry data but how can I include it in the SMS message? Its not like getting the data from data extension.
When you set your entry event as SF Data, then a data extension is automatically created with the selected object fields on activation. The process for calling data into an activity is the same across DE and SF entry events. The primary difference in this regard is naming convention, so it's likely you'll need to wrap any calls to retrieve those values with . i.e.
SET @lead_id = [Contact:Lead:Id]