Who is the provider under question?
If this provider provides the basic functionality that nearly all providers out there handle, then each Voip Account can handle a number of simultaneous calls. Imagine a VOIP TRUNK - Normally they can handle 10 simultaneous calls/account.
This means that you will encounter a BUSY only when you have 10 simultaneous calls already active on each account and you try to make the 11th which the provider will reject. Also given that the Provider line in 3CX is forwarding to a Digital Rec.
If the accounts are forwarded to an extension, then you have to modify extension settings in the 3CX.
a) Busy Tab - Use Phone status. If you have a SNOM for example as the final destination, you can receive all calls on the same phone.
b) If you leave Busy - Use PBX status, then you can forward the call based on the extension settings you set on that extension. See available options for Busy. You can also forward to an outside number which can utilise the other voip account thus connecting 1 account to another. Remember to include the outbound rule for this.
Also - many VP have a website interface where you can log in to for example to check your credit status, some enable you to Spoof your Caller ID for example, others have Voicemail settings, others have Network settings where you bind your Public IP address to the account. But All have some sort of forwarding features and you can also set the Busy status settings from the provider's interface. Many enable you to control the number of simultaneous calls each account can accept. Note: The moment you modify any routing options in this area, you will be lifting the routing responsibility Off 3CX and giving it all to the Provider. Then it is up to the provider to satisfy demands.