Why or why not? If so, which are the best providers for this configuration?
That is very good question, for SIP we highly recommend using SIP security on the firewall this prevent issues with SIP attacks resulting in unknown phone calls being made from your PBX causing a high phone call bill that you didn't generate however in some cases when working with the Fortigate firewall and older versions of PBX you may need to disable this function its called SIP ALG (Application Layer Gateway) this usually cause problems with SIP VoIP phones registration and call processing but you need to make sure you only allow the PBX to only communicate with the specific voip server for security.
Regarding to NAT Traversal it is mostly used when you have devices that are not SIP aware and the firewall is then used to NAT the actual ip address of the SIP phone when communicating with the external ips or VoIP servers,with the use of security policy this can ensure that the voip traffic is also secured by the firewall.
If you have SIP phones which need to access PABX from wan (internet) you need forward sip from wan to LAN PABX.
If you have more than 2 devices that need to share the same internet connection. You have to enable NAT.
NAT support devices are following devices
1. Any Broadband router. ( Cisco, D-link, TP-link, Linksys, Asus,…etc)
2. Firewall /Router/VPN (Fortigate, Cisco, Sonicwall, Paloalto, Watchguard….etc)
My preference is FortiGate. It supports SIP, NAT Configuration & VPN in the same appliance device. SSL VPN is free of charge included with the devices.
SIP is a protocol used for session management in VoIP or video communication, On the other hand NAT Traversal is a technique used to maintain connectivity over networks where NAT is used. You are probably looking to implement VoIP in your network if I'm not mistaken. There is no choice here because some VoIP devices require the implicit use of SIP protocol, That is what they use to initiate, manage, and terminate sessions.
While there are some vendors that use their proprietary protocol, SIP like protocol is necessary regardless. And about NAT traversal, if you have a NAT device or a firewall that implements NAT in between or as a gateway NAT traversal must be used to make sure your communication works because in VoIP communication the client also acts as a server, meaning the communication has to be both ways. When there is a NAT in between NAT masquerades the original IPso there is a probability that the communication may fail. However, some VoIP solutions have their own mechanism to bypass NAT and maintain communication while some require NAT Traversal to be configured on the firewall.
NAT, ISP normally provides one public IP to subscribers and for many devices to connect on the internet this single public IP address is shared among them. Traversal technique is to do UDP encapsulation to allow traffic to reach the destination device which does not have a public address.
SIP traversal is mainly used when we have SIP phones which are registered to a remote IPBX, to keep the connection live and keep signaling link between the phones and the sip registrar, SIP traversal comes into play.
I have mainly used this SIP traversal option on Cyberoam / Sophos firewall and believe me it works like a charm.
"For SIP it is best to use SIP proxy technologies (for example Cisco CUBE). It is much more secure and has an advantage in that the external traffic stops in the Gateway which in turn will make another connection to the inside. Thus, any DoS attack will only affect the gateway and not the applications and internal communications.
Another advantage of the SIP proxy is that reconfigurations and transformations specific to incompatible SIP sessions (DSP transcoding) can be performed on the gateway.
On the other hand, NAT traversal has the advantage of being a cheap solution, with only security facilities and not for improving the parameters of RTP and voice signaling."
SIP is a VoIP telephony protocol, it is not a firewall configuration. In the firewall, the only item you can configure is SIP ALG in disabled or enabled mode. You can configure NAT Transversal when you need to implement site-to-site VPN where the VPN hub is behind a router.
The business need should always be part of the equation if you have a business need for SIP in addition to permitter security then using a firewall with SIP protection such as a FortiGate running version 6.x.
If you only need a SIP gateway then there are several dedicated gateways that are available, but I am not familiar enough to recommend a brand.
There are lots of blogs on this topic. That will be your best resource.
You should configure NAT on your firewall this is for Securing the Internal network (LAN) from external network (WAN),
SIP is a protocol for Voice over IP in digital networks.