Not sure if this is correct or not, but I've got the impression that compared with other verticals, pay per call requires a much higher budget to do proper testing. The typical payouts are higher than those in other verticals and keywords are fairly expensive too.
Would it be a good idea to make money from other sources, save a much bigger budget before trying pay per call?
I'm not sure if it's true that it requires a bigger budget, that may be due to the cost of the clicks and the payouts.
For example, on a campaign like Livelinks you could spend $100 and see 20 conversions. On those 20 conversions there are likely a few keywords that converted more than once, so you know they are decent keywords. So, you could spend less and learn what works or doesn't.
If you promoted a campaign with a higher payout, that converted at a lower rate, like auto insurance, you'd need a bigger budget to figure out which keywords worked versus don't. If you spent $100 you might have 2-3 conversions on 2-3 keywords and you'd be less likely to know if those are consistent winners or not.
There are certainly more campaigns like auto insurance, than there are Livelinks, in pay per call.
I would suggest picking one thing and focusing, versus trying to make money elsewhere and saving budget for pay per call later. If you have an interest in pay per call or feel you have an insight/advantage on a particular campaign, then go with that!
There are several opportunities in online marketing, but focusing on one path and following it through is the way to success.
Best of luck!
-Mike