I have done that for ages. Especially when they are both even and have multiple similar properties! I cope with this by writing the number on a piece of paper with the digits separated into groups. E.g. I will do this to the Anglian Windows phone number since it's a public post and I vaguely associate double glazing with cold calling though they don't necessarily do it:
0800 because it's free, 9 ach nein, 541 5=4+1 203 twins - oh wait - triplets

If I didn't do this I would never phone a right number again except that my phone remembers them for me!