Short answer
Usually yes. For VoIP to work reliably, SIP ALG must be switched off on your firewall and the RTP media ports must be open. FirstNet checks both as part of its readiness review before any voice service goes live.
In detail
Why these settings matter:
- SIP ALG (Application Layer Gateway) is enabled on many small-business firewalls. It alters call signalling, which typically causes failed calls or one-way audio.
- RTP carries the voice itself. If the RTP port range is blocked or restricted, calls may connect but the audio is cut.
- Symmetric NAT can also disrupt calls, so FirstNet runs a NAT type test to flag it.
Before go-live, FirstNet records your firewall make, model and firmware, confirms SIP ALG is disabled and tests the NAT type. For SIP trunks, the plan also covers port and IP whitelisting, NAT traversal and whether a session border controller is needed for an older PBX. Where your PBX supports it, FirstNet SIP Trunks use SIP over TLS for signalling and SRTP for media encryption.
If your current firewall cannot be set up correctly, FirstNet can supply a managed firewall service from Sophos or Fortinet.
Source: FirstNet Voice service page →
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