You can take care of all your phone and message needs with one product. One2Connect offers a One Number Service that offers a toll free number for you to receive and check all your voicemail with no per minute charges and you can use this service as like a calling card and get a low rate of $.07 per minute anywhere in the US any day, anytime. Call 1-877-495-7844 for more info.
I would suggest that you check out the Escapees RV Club. They are the legal address and mail forwarding service, as well as many other services to about 10,000 of us who are fulltime RVers. You can find then at www.escapees.com .
You might want to consider NOT forwarding messages, but changing you landline over to a cell phone. We have been happy with the "America's Choice" plan from Verizon Wireless. there are only 3 places in the country that we've been without service, and usually only a couple of miles to get "out of roaming" and into either "Verizon" or "extended nextwork" both which are covered in our minutes of use (no extra charge).
You can get the number in the area in which you live so that it remains a local call to the people in the area you are from.
We purchased the data cables for cheap off of ebay, so we can access the internet via the cell. Our phone is broadband compatable, so many areas we have access at up to 128k. But, if we are not in such an area where that is available, we always have the option of 14.4k. So, for those cases, we have a separate e-mail address for our people to send pictures to, so that when we accessing at a "slower than dirt" speed, we won't be spending the majority of the time downloading pics until we are somewhere with high-speed or wifi.
The key to choosing a wireless plan is getting the amount of minutes per month that will be closest to what you will use without going over.