This is just a mad idea, it is a protocol for missed call modulation (MCM), it is more a technical joke than a feasible solution.
The Idea:
Cell phones ( as well as regular phones ), have the capability of making missed calls, calling a number without the call being answered. This can be used to transfer information. Many phone companies, at least in Catalunya, give away free phones to get new costumers, so this would be a nice way to exploit the communication companies.
As with any kind of modulation the receiver must be able to identify the bits and the timing. Timing is a hard problem with missed calls, the time can widely change between each call, so some control will have to be added.
Design #1:
This design tries to maximise reliability, increasing the number of phones used. I will discuss the following scheme, step by step.

This shows a simple one way communication, the transfer begins when one of host’s 2 phones rings, if TX phone calls phone 0, this represents a 0 bit, when phone 1 is being called, it represents a 1. After one of the phones has received the call, host 2 issues an ACK to Host 1. Now the control seems quite useless, but in the following example it’s use can be seen.

In this scheme it can be seen the multibit ( 4 bit ) full duplex communication design, more commonly instead of using 4 bits as Data width, 8 bits would be used. The control is really simple, communication starts when one of the Data phone receives a call, after a full word has been received the control phone issues an ACK, if after some time the word has not been fully received the phone will issue a RETR and the word will be sent again.
How many phones are needed for an 8 bit full duplex communication ? 8 for receiving 0’s, 8 for receiving 1’s, 8 for sending and 3 for control, all this on each host. That makes a total of 54 phones.
Which speed could be reached ? This is just an approximation, done without any testing, so it would not have to be taken seriously. I suppose that a phone call takes 3 seconds to be accomplished before timeout. So this would make 3 seconds for transferring the 8 bits, and then 3 more seconds for the ACK/RETR. This is a byte every 6 seconds. So the speed is 0.16 Bytes/s for each up/down channel.
Design #2:
Another function of cell phones, I am not sure but I think this does not apply to fixed phones, is while receiving a call being able to cancel it, the other end notices so.

Communication starts when sender calls the START phone, after that, receiver calls back it’s sender bit, if the sender cancels the call, it is counted as a 0, otherwise if it does more than 3 rings it counts as 1. After the bit is received the receiver calls the ACK phone.
This technique would also extend to a 8 bit width and full duplex, using the same idea as before by extending the DATA phones and leaving the control for full words.
How many phones would this require ? For an 8 bit and full duplex communication, 8 phones for TX, 8 phones for RX, 3 phones for control, and this for each host. That makes a total of 38 phones.
The speed, however would be much lower, 3 seconds as starting signal, then 5 seconds for cancelling or not the call and 3 more seconds for the ACK/RETR. This is a total of 11 seconds for each byte. About 0.1 Bytes/s for each channel.
Improving the speed:
By transferring in parallel multiple words we can greatly increase the speed of the transfer, then adding block control phones. How many phones would we need to be able to maintain a voice conversation ? Using design number one and not counting the block control timing, we would need to reach a speed of at least 3 Kb/s this means we need 19200 parallel word units, each word unit contains 54 phones, so we would only need 1.036.800 phones…
Using design number 2 we would require 33796 parallel word units, each word unit contains 38 phones, so this makes a total of 1.284.248, this is more than with design number 1.
Advantages:
As it has been noticed speed is not the advantage here, but it is free and gives a method for hiding messages, it would be really hard to sniff this kind of connection.