We supply various equipment. Usually it is equipment made by third parties, such as ZyXEL, Billion, etc. They will often include features such a NAT in the equipment, which you can use. NAT is not something that has a defined standard to which it works - it is a bodge, and so the quality of the NAT features in such equipment is not something we can guarantee. We supply them as is in that respect, and you are welcome to ask the manufacturers for details of what they do with NAT before you purchase (but don't expect a detailed reply). Our recommendation is, as always, not to use NAT at all. Using any sort of NAT, whatever the manufactuters claim, is likely to cause problems with some protocols, especially SIP.
We also supply FireBrick firewall/routers. These do allow basic IP and TCP/
IDP port mapping if you want, and as such can provide basic NAT features. In the case of the FireBrick there are specifically no ALGs (Application Layer Gateways) and any NAT or mapping is purely at the TCP/UDP level. You can configure timeouts for sessions in the configuration for FireBricks which can help. Again, our recommendation is, as always, not to use NAT at all. If your VoIP devices have fixed IP addresses then firewall rules on a FireBrick are simple to set up to allow correct SIP operation.
===NAT is evil===