Nextone MSx is a very powerful SIP/H323 soft-switch. Works like rock, but expensive.
PortaBilling is a decent pre/post-paid billing system (the best of cheap low-end billing solutions)
PortaSIP is a collection of SER/Asterisk/Vovida which provides SIP-based services and integrates them with PortaBilling.
Other than that, it is impossible to suggest not knowing the details of your problem.