“How do they get the phone numbers of frum people??”
Unfortunately, frum yidden are no more immune to the same issues as the general population. Many frum mosdos such as shuls and yeshivos maintain contact listings with names, addresses and phone numbers for members, students etc. Also, fund raising groups frequently exchange or sell their mailing lists with the telephone numbers of those who have previously donated. Even some frum businesses keep ohone listings of customers which are not always maintained on a confidential basis.