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In an interview with Bill Gates on the Microsoft "unified communications" the reporter asked Bill Gates "Are the PBX makers going to dig in their heels? Do they have any alternative to what you are offering or what Cisco is offering."
I don't quite catch the meaning of "dig in their heels" in the question. Does it mean the PBX makers don't want to change their business models?
I think you have caught the meaning correctly. If you dig in your heels, you try to resist change which is being forced on you. Like a donkey literally digs in its heels to make it harder to pull, so the PBX makers figuratively dig in their heels.
"Dig in their Heels" 
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