[SR-Users] want to go mobile, common issues ?

Hi everyone. I have enjoyed talk about realtime of last FOSDEM, talk about K. was good. I want to know how do you solve sip dialogs with mobile? actually I know REGISTER is important to authenticate and this request generally last 1hour until then you have to register again. How to solve that for mobile telephony (and what about idle) ? do cellular phone use this schema ? you have to go UDP ? does TCP comes better to handle mobile telephony? I want to ask about IMS as well :-) I heard many good things about Image Multimedia Systems, but in concrete terms what is it? Does IMS target mobile only ? i heard about Boghe, it has many more parameters regarding DNS SRV and extra options. What's so different from my traditionnal Linphone ? thx you