Why do you think, that the v_phone string will be replaced by the variable value?
And you have tried to debug the v_phone string, and what you have seen there has the proper format accepted by the gateway?
Try:
string baseurl = "http://sms.ssdindia.com/sendhttp.php?user=rajnamm&password=298426&mobiles="+v_phone+"&message=hello&sender=test";