The best way is to register the Guest that post an ad, using only the dj-classifieds add form. Using only this form user can register and add his ad at once with one form. This is by far the best and fastest way for the Guest.
SMS payments are depending on the country you’re in. You would need to specify exactly what payment method (payment gateway/provider) you mean.
A serious option for sure
This feature will be presented in version 3.7.1 , after releasing the main update of version 3.7
Yes this is a great feature