Hi Tsveti,
Welcome to post in our forum.
Regarding your issue, you could see the phone numbers in the contact card of windows phone device, but it is invisible on IOS device, so the issue may be come from the SFB mobile client side.
Would you please tell us did the issue only appeared on the specific IOS device, did you try to use another normal IOS device test if there is the same issue?
Did the issue only appeared to the specific user in your environment?
I did a test on my iphone6 (10.0.2), my SFB version is 6.8.0.210, and SFB server 2015, it works fine, I could see the phone number in the contact card.

Please try to reinstall the SFB client on your IOS device and make sure it with the latest update and test again.
If the issue persists, please backup your data and try to reset your IOS device and test again.
For SFB server, make sure it has the latest update.
Alice Wang
TechNet Community Support
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