var user = from u in Users
join uc in UserClients on u.Id equals uc.UserId
into myuserwithclient
from m in myuserwithclient.DefaultIfEmpty()
select new {
u.Id,
FirstName = u.FirstName,
LastName = u.LastName,
UserId = m.UserId,
MobileNo = (m.MobileNo == null) ? "N/A" : m.MobileNo
};