I have quite a bit of experience with call tracking providers. Mongoose is quite good and one of the leaders at this point. They have been innovating left and right, their rates are a steal, and they integrate with all the major PPC campaign management platforms like Kenshoo, Marin, etc. if you use those. They can do some really cool stuff with speech recognition now too where they can listen for certain phrases and trigger conversion actions/pixels as a result--even integrating with Google Analytics.
That said, other providers can be competitive in other ways. Clickpath has very upfront pricing which can be a relief after dealing with the Mongoose rep (I feel like I'm being sold a used car when I'm on the phone with them and they are persistent buggers too--have another VM from the sales rep today).
Telmetrics is ok (for a Canadian company), but their rates are VERY high and they are behind the times with innovating. Also, their Perspectica interface through their site is absolute crap and I dread whenever I have to pull a report from it. Mongoose's is like going from DART to Kenshoo (if you ever had to try to make a lot of campaign changes at once in DART without a bulksheet you'd understand).
I won't even consider Google's Call Metrics solution or whatever the fuck they're calling it. $1/call regardless of whether someone clicked the ad? Are you fucking kidding me? Mongoose can charge as little as $1/line/month with stupid low per minute rates. Why the fuck would I ever consider paying $1 when I have that option? Also, depending on the sort of volume you drive, Google's solution doesn't even give you details on the number that called that you get from all the other providers. Hell, I'd even create a Google Voice number to use instead because its free and you actually get caller details (like the actual number that called).
Feel free to ping me if you have any detailed questions you'd like to go over. I deal with this shit day in and day out at my agency and have spoke to many of the providers out there and worked with a couple.