If you’re looking for a SIP server that can handle concurrent calls, I’d recommend www.callwithus.com which I have tried. Some of its outstanding features include:
- There is theoretically no limit on the number of concurrent outgoing SIP calls. You can have as many concurrent ougoing calls as you want, subject to your available balance. Each outgoing call requires a minimum balance of 1 USD, so to make 10 concurrent outgoing calls you’ll need at least 10 USD available balance. I have myself tested up to 10 concurrent calls using PJSIP with no issues.
- Good web interface. A lot of things can be customized from the web interface. You can check rate, perform a call via callback between 2 phones without the need for a SIP client. You can also set a caller-ID which will be displayed as your number when you call. However, caller-ID is not reliable for countries other than US and Canada. In my tests, I call a Singapore number. Out of 10 calls, the caller-ID only showed for only 2 calls.
- Support for DID (Direct Inward Dialing) number. You can purchase a DID number for people to call you on your SIP phone. Max. 4 concurrent DID incoming calls.
- Low calling rate. Rate to Singapore is only 0.02 USD/minute.
- Easy to top-up credit. You can purchase credits via Google Checkout (supports VISA, Mastercard and other major credit cards). It’s on a prepaid basis and not a subscription. You don’t have to worry about monthly charge to your credit card; the service won’t renew automatically – but remember to topup when you run low on credits
- Good customer support. I received a reply within an hour upon submitting my query.
To me, the most useful features is the unlimited number of concurrent outgoing calls. Very few other SIP service providers offer this.