Simple Asterisk Forwarding Setup

[RobT]

I'm considering setting up asterisk to implement a simple forwarding system and would appreciate any hints, or advice on things to watch carefully.

Suppose I have 3 SIP providers providing DIDs for incoming (to me) calls. I want asterisk to register to each of those SIP providers and foward any incoming call to my account at a 4th (IAX or SIP) provider. I would prefer that asterik doesn't do any codec transcoding. (To the extent possible, I'd prefer asterisk to stay out of the voice path, but my reading suggests that may not be possible.)

I'll be implementing this on a Unix box, so no @home, etc.

For my purposes I'm happy to assume that the first 3 SIP providers will be incoming only and the 4th (IAX or SIP) provider will be outgoing only.

All suggestions appreciated.