Call aggregation or simultaneous ring are possible if your carrier plays nicely. Most don't; I'm not sure why, but I'm not familiar with too many companies who have this at the core of their value proposition.
Google Voice does offer this; formerly known as GrandCentral, the service allows you to present a single number, which
- can receive text messages (SMS) and deliver them to all the capable devices and/or e-mail
- can receive voice calls, and ring multiple devices allowing you to answer the call on the most convenient one
- switch from one phone to another mid-call, as well as record calls
- provides a single voicemail service, which can forward the voice message via e-mail and transcribe it to text