Disable SIP ALG of the router
Many of the problems related to calls to / from VoIP operators are due to the corporate router and firewall configurations used by the PBX to send and receive audio streams. The main cause is related to the ALG (Application Layer Gateway) or SIP / VoIP ALG function which, by analyzing the content of the SIP packets (on the standard port 5060 *), modifies the header, compromising the correct routing of the packets. This leads to the already known problems of no audio in at least one of the directions of the call.
In these cases (practically 100%) the SIP ALG functions must be absolutely disabled. In cases where it is not possible because the router does not allow it, you can try to make the SIP gateways (especially VoIP operators) and remote phones work on a non-standard SIP port, such as 5061, but not all operators allow to use this door and it is necessary to ask them expressly if they allow the use of the door or not.
In the event that the router allows the deactivation of the SIP ALG functions and you do not know how to do it, you can try to follow the instructions at this link where the instructions for some router models are indicated.
In the unfortunate case in which neither of the two paths can be followed, it will be necessary to change router, or hope that the router applies ALG only on the standard SIP 5060 port and therefore you can opt for a change of SIP port of the VOIspeed server, but in doing so I will be necessary.