public class User
    {
        public User()
        {
            Name = string.Empty;
            LastName = string.Empty;
        }

        public void SetInnerProperties()
        {
            S = string.Format("{0} {1} {0}", Name, LastName).ToUpper();
            T = Phones == null ? string.Empty : string.Join(" ", Phones.Select(x => x.Phone));
//x.ToPhone() removes white spaces and non digit char
        }

        public string Name { get; set; }

        public string LastName { get; set; }

        public List<PhoneNumber> Phones { get; set; }

        public string S { get; set; }

        public string T { get; set; }

        public override string ToString()
        {
            return S;
        }
    }

public class PhoneNumber
    {
        public string Phone { get; set; }

        public PhoneNumberType PhoneNumberType { get; set; }
}

users.ForEach(x => x.SetInnerProperties());

if (isPhone)
            {
                var reqPhone = req.ToPhone();//remove white spaces and non digit char
                var resultPhone = users.Where(x => x.T.Contains(reqPhone)).Take(10);
                return resultPhone;
            }
            var reqUp = req.ToUpper();
            var result = users.Where(x => x.S.Contains(reqUp)).Take(10);
            return result;