[digi]

You can use MagiCall to prefix digits to outgoing calls, and it can be setup to do it for specific numbers, phone book entries, groups, etc. Its all setup as rules and the rules can be turned on/off easily, can even be scheduled to only work at certain times.

MagiCall is also the best app I've ever used for call blocking as well, I tried Call Firewall and it was very slow to respond, my phone would still ring once or twice before the software caught it. With MagiCall, the only way I would know it had blocked a call was if I turned on the notification.