I was stymied as well but I found it, once you are on a call you pull down the menu from InternetCall (like File menu on a PC) and under Internet Call is "DTMF dialing". That pulls up a dialing pad. DTMF will then work. Rather unintuitive but at least it works. I use dialing codes on the PBX and since there isn't an * on the regular pad I had to create a Contact entry in order to dial the codes such as voice mail (*97).

The SIP (Internet Call) client needs some work but it is good enough that I use it as a primary phone. I hope to see it develop further (put the * on the regular dialing pad etc)

As for SIP not being newbie friendly, that's the nature of an open standard. Having a new account being setup from the NIT on a remote server isn't realistic except for proprietary systems. Even jabber accounts won't always work (you can't setup a new account on my jabber server, that option is off at my end). We're all newbies at times so why be scared off from doing one extra step...