CPH_BLOGGER  I have read some of the other reviews on this site, which are highlighting the cost of the product as a negative. There most definitely are free alternatives out there that will work for some people, however I have had bad experiences. The only SIP client that would work with my VOIP provider was SJPhone and the software had not been updated since 2005! Therefore I was experiencing constant crashes mid-call (very irritating). I spent hours looking online for a programme that integrates with mac os and runs smoothly and isoftphone is the only product that matched this criteria. The softphone does not have too many features that over burden the mind with complex tick-boxes, but simply does the job and offers the flexibility that is needed. I particularly like the ability to change the ringtone, but prefer the standard one, as its very noticeable! I use a macbook air and find that the slightest amount of CPU expenditure causes the fan to start, so the fact that isoftphone uses very little memory and is hardly noticeable on the running of my mac is definitely welcome. The real question with SIP phones is whether or not the call quality is up to scratch. I have tried too many pieces of SIP software and was quite frankly growing sick of the sight of them. I have found that none surpass isoftphone in terms of reliability and general call quality. As with any SIP application running on a laptop or desktop USE HEADPHONES. It makes a massive difference to the general quality. Sorry to all those who are thinking, "what an idiot, we all know that".   
(Version 2.5)