The display of a caller's number at the choice of the caller is intended to be used on a per call basis. Exactly as you do now, key 1470 immediately before the number.
With Speed-Dial the number is replaced by a * * quick code but in its present form Speed-Dial doesn't allow the storing of the 1470 prefix before the stored number.
What will work when you call the number is to key 1470 * * 3, for example, where the Speed-Dial number you want is * * 3.
What we'll do here is ask the TalkTalk team to ask the MyAccount Team and or relevant stakeholder to see if it's possible to allow the storing of a 1470 prefix (or 141 prefix) as part of the Speed-Dial number.
I am genuinely curious as to the thinking behind this. I imagine the purpose of a Speed-Dial number is for those frequently used. In which case presumably known to the caller. In which case why would you wish to withhold your number from them?