As Mark mentions, the most difficult part of attacking your password in this way would be gaining access to capture the numbers entered during your phone call. If an attacker wants to tap your phone line (assuming non-mobile) they need physical access to your house or demarcation point outside your house. Depending on where you live it might not be difficult to access your demarcation point to install a tap, but it would require the attacker knowing where you live. Someone with telco access could also do this, but that's generally much more difficult hack.
They would then attach a device that records DTMF tones so they could later watch for a call to Fidelity followed by your passcode. Capturing the DTMF tones sent when you dial and converting them back to their numeric representation is pretty easy.
Alternatively, if you dial up Fidelity using a smart phone it might actually be better for the attacker. If they could trick you into installing a trojan on your phone this could monitor your calls and capture any digits sent. Of course the trojan could also capture your exact password if you log into the Fidelity web site using the phone's browser.
I've been told by another customer that you can log into the Fidelity web site with the same numeric password you use over the phone. If an attacker just wants into your account then no brute forcing would even be necessary.
Assuming the hacker did want your original password they would have to compile the possibilities. 1 and 0 only represent numbers, while 2 - 9 represent either a number or one of three to four letters. Since they'd want to distinguish between uppercase/lowercase letters that gives us 9 total character possibilities per key.
Worst case scenario (meaning no 1's or 0's were used, and all 7's and 9's) for an 8 character password would then be 9^8, or 43,046,721. It's unlikely a person would really use all 7's and 9's, so that number is likely to much lower. They might also prioritize that pool of possibilities with words or other common password formats guessed first to improve their odds finding it sooner.