Yes - both you and your client will be given access to each other's telephone numbers and email addresses.

To find your client's contact details, select their name from your Clients list (or click on their name from your Calendar or Dashboard etc.). You will then be shown the client's contact details at the top of the page underneath their name :

It is entirely up to you, but you may wish to discuss boundaries and expectations around communication with your clients when they first start sessions.

Contact should usually be kept to a minimum for clinical reasons, unless your therapeutic approach requires more extensive communication than usual. 

Exchanges outside of therapy sessions should normally relate only to organising follow-up sessions, accessing your clinic/online sessions, or sharing recommendations for supporting therapeutic material, etc.