Fortivoice - Random Interruptions

Recently, I have changed the telephone system in our company. We have chosen the FortiVoice 300E-t, with some FortiFones 480B and some FortiFones 580. Unfortunately, there are some issues with the calls, whether internal or external, and the problem mainly focuses on those using the FortiFones 480B devices. During the calls, there are random momentary interruptions or it seems like words are being lost.

The telephone line used for the calls is an ISDN PRI E1 and it is connected to the FortiVoice 300E-t. The network is divided into a data VLAN and a voice VLAN; the telephone devices correctly receive IP addresses from the voice VLAN and have communication with the telephone system. The telephone system and the telephone devices are connected to FortiSwitches (148F-FPOE), which are managed by a FortiGate 60E. The telephone system and the phones have the latest firmware installed.

Can someone tell me if they have encountered a similar issue or if there is a way to find out what is really causing the problem?

Thank you very much.