First of all, I'm not sure whether the question is answerable at all without a more detailed scenario.
So we have:
Ca²⁺ excretion increasing with excess protein
This seems to be linked to elimination of excess acids rather than directly to protein according to http://jn.nutrition.org/content/141/3/391.long.
https://www.ncbi.nlm.nih.gov/pubmed/15546911 finds the additional excretion coming from the increased intake under high protein diet.
Ca²⁺ uptake/loss in bones (which is typically the main point of the Ca uptake/loss discussion, and bone is a large researvoir of Ca in the human body):
bone (re)building needs:
(we're talking here high/excess protein diets, so let's assume this is not a limiting factor)
- mechanical stimulation (excercise)
if you are in a situation with (primarily) net loss of bone because of any of the 3 ingredients to bone growth above (e.g. lack of excercise even though Ca and protein are available), you'll observe a net Ca loss, because Ca released from the bone that (naturally) broken down is excreted.
Ca²⁺ uptake from food depends on
Taking those points together, we may construct 2 extreme lifestyles that would react quite opposite to the linked intake of protein + calcium in milk:
On the one hand, imagine a sedentary indoors couch potato. No sun (low vitamin D), no excercise (no bone growth stimulus). Without adequate vitamin D levels, the Ca in the milk (although in principle bioavailable) is not even absorbed in the gut. Even if it were (or: even the little that is) is not used for net new bone growth as the excercise stimulus is missing. With a high protein diet (or, a diet causing excess acid urinary excretion), Ca is lost. It may have been lost even without a high protein diet because of bone loss due to lack of excercise. Or, with a low excess acid diet, bone would have been maintained even with the low level of excercise.
On the other hand, consider a physically active outdoors person. Excercise gives a bone growth stimulus, sunlight does its share in supplying vitamin D. In that scenario, milk gives the protein as well as the Ca to actually grow bone. Result is a net gain in Ca, even though some Ca is excreted via urine.
Slightly off topic: this whole scenario feels a bit like a 101 in how to lie with statistics without actually lying: we have at least 4-5 factors that need to be at the right setting (and for 4 of them it is comparatively easy to be off) in order to achieve bone growth/net Ca gain. Now any study looking at a single factor will not find an effect unless they make sure all other factors are right. And if one factor is worse with the treatment group than the controls, a study may even seem to observe the opposite of what is really going on.
I tried to find numbers for Ca²⁺ excretion per gram of protein. A rough guesstimate based on http://jn.nutrition.org/content/128/6/1051.full would be up to 200 mg Ca²⁺ urinary excretion for 150 g of protein consumption. Based on that rough guesstimate, food with a ratio > 1.33 mg Ca/g of protein would be considered as "gaining Ca²⁺".
https://en.wikipedia.org/wiki/Milk#Nutrition_and_health gives cow milk with 30 - 35 g protein / l and 1200 mg Ca²⁺/l, yielding a Ca : protein ratio of > 30 mg Ca/g of protein. Even assuming just the average absorption rate for Ca in food of 30 % (http://ajcn.nutrition.org/content/35/4/783.extract - didn't find numbers for milk, though it is frequently cited as highly bioavailable - but then it will also depend on other factors) that leaves almost an order of magnitude to the side of more Ca uptake than increase in excretion.
So overall, this back-of-the-envelope calculation is in favor of milk being a net source of Ca. Note however, that not all dairy products have the same Ca:protein ratio as milk. E.g. Whey has even more Ca:protein, and consequently the cheese of which whey is a leftover/side product has a somewhat lower Ca:protein ratio.