I think that we may have two different MACs being discussed here.

Your router (or PC, or network printer, or anything with a network connection on it) will have a MAC address. In this case, MAC stands for Media Access Control and it is a unique number that identifies that specific bit of network hardware. It typically will look something like 00:BE:8F:01:FF:CA (or some other collection of a dozen hexadecimal digits) and may well be written on a label somewhere on your router, network card, wi-fi adapter, PC, laptop, etc.

The MAC that you get from an ISP is a Migration Access Code. It is usually a 17 or 18 character alphanumeric code and this is the thing that you need if you want to change broadband providers. This will normally have no relationship at all to the MAC address on any of your network hardware, since there isn't really any meaningful correspondence between the two. (If your ISP tells you otherwise then they're either one of the small number of ISPs that forces you to use their own hardware to access their service and they are using that network MAC address for some purpose or other that it wasn't really intended for, or they're just plain telling fibs.)

If you want to get a MAC from your ISP, there is a useful website called www.maccode.org.uk that explains it and gives the correct number to call for most ISPs.

If you want to know the MAC address for a piece of hardware, on the other hand, check its labels first. If that doesn't work, you'll need to check the manual and find out how to view or print out the network configuration for that particular bit of hardware.

Here endeth the boring, geeky bit...