If you want anything more than just call forwarding, have a look at solutions not based on Asterisk. FreeSwitch and Yate are quite good. In my experience Ast. is like Windows - people complain about it, but continue to use it because most other people use it... and it's not that good compared to others. If you do want only forwarding, have a look at sip proxies instead (like Opensips) - that's more reliable and can save you one hop on the media path.