If you want a way to answer your office phone on your mobile phone, or want your voicemails transcribed and either emailed or sent to you via text, consider Ribbit Mobile. Ribbit appears to offer very similar services to Google Voice and as of right now, is also free.
Once you create an account, you can tell Ribbit which phones to ring when someone calls your number. Ribbit can also transcribe your voicemails and send them to you when you cannot, or do not want to, answer your phone.
Sending the calls to various phones is not that exciting. If you are a solo practitioner, you might use your cell phone as your business phone. Most phone companies, including wireless companies, allow you to forward calls to other phones. Theoretically, the advantage is that you can use Ribbit to have all the phones ring at the same time.
Voicemail transcription, on the other hand, is a handy feature. For solo attorneys, you cannot always answer your phone, meet with clients, and complete legal work. When you cannot answer the phone, transcriptions can help you do quick triage on your calls. Google Voice is somewhat notorious for having poor transcriptions, so perhaps Ribbit can do better.
Ribbit is currently soliciting invites for its beta phase.