"You need to have an unlimited incoming call package"
So that the call back doesn't cost extra.
"Simply dial the ComWave Unlimited Mobile Access Number"
"Your call will not be answered"
When you call the access number you are actually dialing into a system that simply extracts the calling party number from the attempted call connection but never actually connects to the call, rather it disconnects as soon as it has the data it needs. No completed call, no charge to the caller.
"ComWave will then immediately call you back"
Since they know from the above what number made the incoming, never connected call, they now call back to that number using their automated system. Remember, it doesn't cost you if you have unlimited incoming calls.
"Dial Your Number Now"
The system opens up a new call connection where you dial in your number and the system connects the call as normal. Typically, there is a VOIP system in the background.
Many global/international low-cost mobile roaming SIMs use the same concept, except they pull both the calling and called numbers out of the connection data before the call-back so when you answer the call-back the called party's phone will start "ringing" immediately without having to dial the numbers. The disadvantage of course is you have a different SIM and different phone number and it only works for GSM/UMTS phones.