Hello,I do not have an external service of this type
But if I had to do it I would do it with a service of this type Messagenet
Or others like it
Simply by registering on the website it is already possible to obtain a telephone number to call, which will make the connected 'client sip' ring
If the connected 'client sip' was the B4J server, then the call will ring all clients (or some of them) that will be able to answer
With the payment of a little fee, even a minimum of 6 euros, it is also possible to make calls to the public telephone network
Certainly in every country there are several service providers of this type
MessageNet has a special authorization from the Italian communications department to enable it to make phone calls, which means you have to get a permission from your country government to give you the permission to do that.
Important note:
Messagenet is an operator with a general authorisation for providing "telephone service accessible to the public" throughout the Italian territory issued by the Communications Department of the Ministero dello Sviluppo Economico.
Any additional services present on the telephone line destined for number portability will not be maintained.
Reference :
Portabilità numero fisso garantita e in pochi step
Mantieni il numero fisso aziendale e affidalo al nostro team dedicato alla number portability: con 20 anni di esperienza il tuo numero è al sicuro.
messagenet.com
Do you have any other example? because if you want to make phone calls to the public you have to be authorized by your government and you will need a special API from them to be able to do that.
Thank you,
Saif