You can use direct dialing to place an international call from your Mediacom Mobile phone with an Unlimited Plan.  

Dial – 011 + country code + the phone number and press the Call button on your mobile phone.  Long pressing the “0” key when you are dialing on a mobile phone will add “+” to the number and has the same effect as dialing 011.For example, if you’d like to call London, United Kingdom, you will enter the plus sign + (press and hold the 0 key), followed by the country code, and then the phone number. 

In this case dial, +44 20 XXX-XXXX with your Mediacom Mobile phone on an Unlimited rate plan, you would dial 011 (or long press the “0” key to add “+”), then 44, then 20 XXX-XXXX and press the call button.