Hi forum.
I am totally new to VoIP and SIP and need to ask some questions.
I have an account with voipcheap.co.uk and I have been using their PC software for a few weeks now.
I wanted to be able to make and receive calls all the time without my PC switched on so I replaced my USB modem with a Broadband router and bought myself a SIP phone - its a Safecom SDSIP-3000.
I configured the phone with my voipcheap details, username, password, domain voipcheap.com, proxy server sip.voipcheap.com.
I can make calls to normal phone numbers okay (voip-out) and I can receive calls from normal phones via my voipcheap allocated voip-in number. All that works okay.
But how the hell do I make voip to voip calls?
The voipcheap PC software enables you to make calls to other voipcheap users without touching the PSTN - excellent sound quality because its not being constrained by the audio bandwidth of the PSTN.
Lets say for the sake of this conversation my friend's voipcheap username is "wibble".
Using the PC software I can call wibble using my speakers and microphone.
I have tried putting wibble into my SIP phone's phonebook.
I have also tried "wibble@voipcheap.com" and "wibble@sip.voipcheap.com" but none of these works.
(I did tell you Im new to voip and SIP - assume I know nothing!)
Perhaps SIP only supports phone-number based calling? I dont know.
How do I call another voipcheap user without using the PSTN?
If I get them to apply for a voip-in number from voipcheap and I call that number using my SIP phone, can I assume the voipcheap SIP server is clever enough to know its the number of another voipcheap user and route it straight to them without going anywhere near the PSTN?

Can anyone validate my assumptions, answer my questions, or teach me all I need to know about voip and SIP?