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I want to connect two SIP phones to my router, alongside one another.

One connects to my commercial VoIP service provided by my ISP.

The other connects to my business VoIP PBX as a remote extension.

Both have SIP configuration options which would be independently configured to the respective SIP server, either the ISP or my PBX at the office.

My question is, will there be a problem with port conflicts if they're both sitting alongside one another in the network hierarchy?

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It depends, mostly from your (unnamed) router. Keywords "Voice ALG", "H323 ALG". In most cases you'll have two different NAT'ed connections without headache, but - see sentence N 1 – Lazy Badger Nov 1 '11 at 11:57

Typically, yes, you can do this. Keep in mind that your router must be able to open ports to a range of IPs and not just one specific IP where ports must be used by multiple devices. Even on the same port, it should be able to route to the right private IP correctly with an open SIP session. If you are using SIP for both, then there will definitely be some shared ports, but there should be no conflicts. If both phones will be set to connect for their TFTP information separately and know where their gateways are, then there will be no issues with the router as far as getting them to register and initiate/receive calls.

You may still encounter some issues with prioritization and saturation, depending on your circuit speed and routes for the calls. I highly recommend also disabling anything to do with VOIP in your router as it will cause nothing but conflicts with what you are trying to do. It is also prudent to disable some firewalling aspects as well. Basically anything that is labeled as ALG, VoIP, Intrusion Detection and Stateful Packet Inspection should all be disabled.

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