- Disable SIP ALG or SIP Passthrough features if any.
- Increate the UDP timeout to 100 seconds, if it is less.
- Set VLANs to separate VoIP traffic from other.
- Set QoS policies to assure the highest priority for the VoIP traffic.
Note: VoIP traffic is not only what has the source or destination port 5060, there is also RTP (audio) traffic which is on a random, high-number port. To be able to capture this traffic for QoS purposes, you will need to create firewall policies based on source and destination IPs. You could either allocate a range of local IPs for handsets or if that's not possible like in the case when using soft-phones, you should check with NUACOM support which are the public IPs of your cloud account and create policies based on them.
Eir (former eircom) F2000 settings for NUACOM VoIP Service
Vodafone Huawei HG658c Modem settings for NUACOM VoIP Service
Advanced -> NAT -> ALG
Sonicwall configuration for NUACOM VoIP Service