I think what you need to understand is that whilst SIP is a standard, some manufacturers products are "more standard than others" IP telephony is at a stage where everyone is keen to introduce more features/functions and do not always adhere rigidly to the "standards", if there really is such a thing as this. I have read the relavent RFC's and some areas can be open to diferent interpretations so what is "standard"?
You best bet is to follow Matt's sugestion and use products such as Snom, Astra, Polycom, they are on the supported list and you will have fewer issues. I am not saying that every feature will work first time out of the box but they will be a lot less painful than some of the cheaper or non approved phones. I would disagree with using newer Cisco phones on the 3CX even though I support Cisco call manager and other Cisco hardware. The later Cisco phones are a pain to convert to SIP, Cisco are not interested in helping with any issues, their attitude is that if you are not using them with Call Manager - tough!
Good luck
Which brings me back to budget phones like Linksys, let me think for a minute......who owns Linksys???? hmm
