What about 'which is our telephone number?'
Could it be used in situations like looking at a list of numbers on a piece of numbers and asking which of those is the respondent's number?
(I'm a native English speaker by the way)
In conversation co? could be probably understood for some declension of co? Probably understandable , not ok.
Jaki? will be understood when used instead of other genders of cases.
But jaki? and co? are as different as how? and who?