Hi,
I would like to get some opinion on the followings.
I’m deciding on what URI sip address format I should adopt -
whether to use a phoneno@domain.tld
How would you call a username@domain.tld from an IP phone?
Any suggestion or opinions on the pros and cons on the above?
Thanks in advance
No real opinion on pros and cons, although it might be a smart move to have your SIP address the same as your email address. On the other hand, maybe a phone number would be better… Hmm, i see your problem! 
However, i would question the wisdom of taking this approach to telephony - if you’re intending to allow the whole world to connect to your asterisk server to make calls, that is. It’s a serious security issue.
As far as how to do it goes, i answered that question for someone else yesterday… Here:
Could you clarify on the security issue?
The plan is to assign a sip address to a user such that he/she will be able
to receive sip calls. Given that there will be spam or irritating calls,
but the idea is that the user will be reachable through his/her sip address.
On the sip address to use, where there be an issue for a SIP phone to call URI-based SIP address?
Thanks in advance.
Re security, have a read of this thread:
forums.digium.com/viewtopic.php? … highlight=
Re URIs, hardphones generally can’t call URI addresses - don’t know if this is all of them or, if not, what proportion.