Not getting true blacks from my printer who uses this machine

I would look at cleaning the printer glass where your documents are scanned first, if it doesnt resolve problem the printer might need a clean throughout and unless your adept and confident it should be done by someone who is.doesn't take much to upset the workings of these machines.

Your scanner array is bad. This is a light "bar" that lights up to scan your originals before they are sent out. You can test for this by making a copy with your machine. The same scanning function is used to make a copy as it is when you scan to send a fax. If you make a copy and it comes out black then your scanner is bad. This is especially true as when you say that the problem goes away for a short time after turning the machine off. This allows the circuit to cool and the solder connections to shrink and make contact. As the board heats up the metal contacts expand and "open up", causing the scanner to fail.
Not a cheap fix unfortunately.
Good Luck and Thank You for using Fixya.

Vonage is dgital. fax machines are analog

Correct! Its a PDF but its just a picture of the characters you will have to OCR it to use the search feature. Good Luck  E-Copy is a good program for doing this or Laserfish

I have exactly the same problem! did you find a solution or reason???

True. ALL fax machines are analog devices. Internet phone lines are DIGITAL.

There is no way to make it fax reliably.

I spoke to a company called CRC (Tasktron Limited) who are repair specialists for all kinds of business machines. They can be contacted at 01483 776060 or via www.crc-tasktron.co.uk.
After explaining my problem and the amount of use my printer is subject to, they advised me that the printhead was past its useful life. They offered to send me a relacement (£40) which I received today (next day delivery) and fitted in two minutes. Problem solved.

Few questions, are you running true Canon Inks and refills are always a off black not a true canon black!

Have you tried doing nozzel check, does the top black squares all come out ok? if so it could be down to the paper and the inks!
if you use 80gsm paper does it print ok?

Hope this helps

Thats because it's using the other colours to mix what it thinks to be shades of grey. Due to the black ink being pigment based and the colours inks dye base, when printing onto glossy paper the machine will use the colour dye based inks to make up black. 
If you were to print on plain paper, set to greyscale, then the machine will just use the black as the pigment ink sits onto of the plain paper and will not run should the paper get wet.  With glossy paper the ink has to be absorbed into the paper ad the Black pigment ink will not do this so the machine will use the coloured inks.

Sucks but true. 

Question: when last was it serviced?
Do you use org. consumables?

By the sound of it the printer needs a service, the shaft and carriage seems to be dry and needs some lubrication