One potential problem with requiring the registration and provision of a phone number is that a lot of mobile applications now connect to people's cellphone numbers, which means that, by allowing or forcing a phone number to be provided, there are potential ramifications for all kinds of different data joining, which would go beyond the ideal of.... A politician or candidate should be able to have that direct communication with constituents and try to engage them in the electoral process, which I think is really good, but I worry about creating a situation where people unknowingly give candidates access to all kinds of information from dating apps to Facebook to their Airbnb listings.
It's not necessarily that the specific listings are going to show up in a candidate's hands, but I think the possibilities for data joining when you use the cellphone number needs investigation. I'm not saying it's necessarily a bad idea, but it's not one I had considered before. My immediate gut reaction is that we need to be worried about the way different datasets can be connected together.
I think your basic point that candidates need to be able to connect with citizens is an important point and, in particular, candidates who are not incumbents end up at a disadvantage if they don't have good enough data to do that basic contact.