ZUNGA  This new version rocks. I've tried the app in its early stages, but I was really disappointed. Since then the voice's quality has improved, as have the semplicity of use and the options. Plus, the price has dropped down. I've contacted the developer in order to ask for a cross-grade from eyeBeam (that's the commercial version of X-Lite, we have 7 licenses running) and he offered me a 25% discount, besides the normal "quantity" discount you can obtain on the site if you buy more than one copy of iSoftphone. I've also tried SJphone and Express Talk, and - as far as I can say - this is the one and only app developed with the Mac user in mind.   
(Version 2.6070)

CHRISTOPHE PERESS DE MONTLOUIS  i purchased isoftphone almost a year ago.

to date john sheehy/ isoftphone is still unable to provide me the correct setting to use their application: it is by far the $35 best wasted!

DO NOT BUY IT: it is virtually impossible to find a working port!

chris  
(Version 2.5)

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)

UETLI  Even though isoftphone has the best interface by far of all the SIP tools it is the worst one.

I cannot imagine why, but nearly every call with isoftphone is unaccaptable because of Echos and very bad audio quality. Everytime I tried again with other tools like x-lite or telephone and the quality was better by far.

In this rare cases that communication quality was o.k. the call was ended automatically after some time.  
(Version 2.5)

XPLICIT  -> 2 seconds delay in calls,

-> Dial tones very loud (despite developers assurances for a future version, which is published by now, it's not possible to lower the dial tones volume separately from the main volume),

-> at every startup Apples firewall comes up again (not as with other applications) and if you don't allow incoming connections fast enough you won't hear the receiver of your call,

-> every(!) call breaks up after about 35 to 36 minutes,

-> and the developers mostly don't reply to eMails.

These are all quite annoying flaws. Makes the programme look like some kind of hobby project, except for the price.  
(Version 2.3102)

SHAGGYMAC  This is the BEST VoIP application ever! It works really well with ShaggyTalk.com (cheap VoIP provider where you can get service for as little as $19.99 a month)!

Just goto My SIP accounts and type the following

Display Name [ Appears as your caller id ]

User name 5555555555

Password [ YOUR PASSWORD ]

Authorization user name 5555555555

Domain 209.17.160.130

Sip proxy 209.17.160.130

replace the 5555555555 with your ShaggyTalk number  
(Version 2.1019)

GARDEL  I tried this out as a result of the MacZot offer (40% off) and was sorely disappointed.

First of all, the product severely favors Sip Phone (aka: Gizmo Project) and something called CallCentric. Unless you are on one of these providers, trying to get this to work is a nightmare. There are hundreds of great SIP providers out there, why limit it to these two? Is the author gettting paid by them? Isn't this something we should know.

Second, for some reason, this thing is written in such a way that the Mac Address Book hijacks the dialer when trying to do extension dialing on a PBX. The softphone registers fine with the ipen source asterisk PBX, but if I try to dial someone on the same PBX with a 4 digit extension, the softphone tries to find a match in the address book and won't let me dial the extension. In other words, I can cal outside the office, but not within it. Very poorly thought out. This was enough for me to dump it.

Third, in an effort to make it look like an iPhone, this product is cumbersome and counter-intuitive. The icons are small and not representative of what they do. There are way too many buttons on the phone, many of them unnecessary.

Finally, even at $21 (the MacZot price) this softphone seems overpriced, at the $35 regular price it's a robbery. Many better options out there. This one was clearly put together by someone with little clear idea of what the power of SIP and open platforms is about.  
(Version 2.0045)

VEGGIEDUDE  It does not work with my RingCentral account. I have no problem using eyeBeam (commercial version of X-Lite) with RingCentral.  
(Version 2.0030)

SAPPOROBABY  $35?

Two words: Gizmo Project. Another word: Skype.

Not to mention the lack of originality. Copying the iPhone interface.

Cheesy.  
(Version 2.0030)

MAJIDS  I use different free and paid softphones for my voip accounts. Based on my own experience, I can draw some advantages of iSoftPhone over similar products:

1. Very clean and useful integration with Apple Address Book. So there is no need to re-enter your contact and keep updating them.

2. In case of using more than one SIP account, it is very easy to select your desired account for making calls. You can not find such an easy feature in other similar products.

3. Very exciting skin ! (like my iPhone)

4. Very clean sip account configuration page.

5. Faster account logins.

...

end now with even more competitive price of $35

I will try to provide more on this later.

Thanks.  
(Version 2.0011)