I think he meant that he wants to replace his existing Partner ACS with a switch that can support a higher line and station count.
Since phones are often half the cost of the upgrade, a logical solution would be to reuse the phones he's got and upgrade to a Magix. I suspect that this is why he was asking about Partner phone compatibility with Magix in the first place.
I have at least two customers who use Partner sets almost exclusively with Magix, and several that have adopted the digital sets as well, creating an interesting mix of telephones.
It takes some effort, but you can even set the line assignments on the phones the way Partner does (across rows, instead of the bottom-to-top columnar method that Magix uses by default).