VoIP/SIP accounts are not intended to be shared, and most VoIP providers don't allow for this. Instead you are supposed to install a PBX between your VoIP phones/IVM and the VoIP service. The idea is the PBX system is set-up with the VoIP account from your provider as if it is a single phone.
The PBX system then shares out the VoIP line to several devices (softphones such as Express Talk, x-Lite, SIP phones, and software like IVM). Each softphone, SIP phone, or software conencted to the PBX will require it's own extension (which is basically like it's own SIP/VoIP account on the PBX).
Axon is one PBX system you can use to do this. It is free to download from NCH's website ( http://nch.com.au )