Robert,

If you mean start a call from a mobile then x5 has not yet implemented this, maybe an IDEA post to Incoemdia will help in getting this in the future.

If you mean to an internet calll like skype, then this is available in the action menu,  create a picture goto link option and use internet call in the pop up.

Workaround:

You could use a html code object, and use some code to make your cell call this is found on internet, for instance here: https://code.tutsplus.com/tutorials/mobile-web-quick-tip-phone-number-links--mobile-7667

Goodluck