When you come across people that you wish to connect or stay in touch with in a professional context, don’t proceed to straightaway ask for their phone number. Ask what would be the best way to reach them instead.
Most people do not prefer to share their mobile numbers and most good professionals do have clear communication preferences as well. Insisting that you have their mobile numbers might come across as unprofessional and intrusive even.
Acknowledging and respecting communication preferences on the other hand will come across as utmost professional and show you in the best light.