Upwork User Agreement:

For Users subject to this Section 7.2, Users agree to use the communication services available on the Site to communicate with other Users prior to entering into a Service Contract. You agree that prior to entering into a Service Contract, you (a) will use Upwork as the sole manner to communicate with other Users; (b) will not provide your Means of Direct Contact (defined below) to any other User or another person that you identified or were identified by through the Site; (c) will not use Means of Direct Contact of another user to attempt to or to communicate with, solicit, contact, or find the contact information of a User outside of Upwork; (d) will not ask for, provide, or attempt to identify through public means the contact information of another User; and (e) will not include any Means of Direct Contact or means by which your contact information could be discovered in any profile, proposal, job posting, invitation, or pre-hire communication through the Site’s communications services (including in each case in any attached file), except as otherwise provided on the Site. User Agreement (link).