Once you allow traffic from pbxes.prg, I will be able to use your account, since it does not require a username or password. If you feel like throwing away money, please let me know.
If your provider gave you a tech prefix which is unique to you, then you are ok. Otherwise you can't use those provider who just do pure sip trunking.
You gotta have a unique public ip address, which you don't have here on pbxes.org