Depends on how many calls you are making, how much extra info you want to record, how many people need to access the system at the same time, how secure your system is, what you might want to do in the future, and last but not least, how much you want to spend.
Where are your phone numbers coming from? - do you need to import prospects from a list?
Are you checking against the TPS list, or is the data already clean?
Do you hold any personal info - do you comply with the Data Protection Act?
Do you want to do any follow ups? (as Jeff said)
You can tweak a custom system to do what you want - but you have to know what you want.
You'll only use 20% of an "off the shelf" system, but it might give you ideas of extra things to do.
And how are you managing people that you do want to contact again?
is this a separate system?
Whichever way you go, two golden rules:
1) make sure you can always get to the data
2) make sure you have regular automated backups