It's possible, but Mpix is trying to protect you from yourself. Your camera's 8 megapixels make an image with something like 2448 × 3264 pixels. That means that when you print an image 16 inches wide, you end up with about 150 pixels per inch. If you look closely, you may see some blockiness, and the phone camera's image quality probably isn't such that it stands up to high scrutiny. General guidelines suggest 300 pixels per inch for the best quality, and it looks like your print service is enforcing that. That's because they don't want you to use their service and then complain about poor results even when it's not their fault.
However, if you understand the limitations and aren't expecting more than is possible, you may be perfectly happy with a 150dpi print. If you normally look at it from a reasonable viewing distance rather than up close, this should be just fine. Your print service may have an advanced setting where you tell them "I understand what I'm doing; please go ahead anyway." If they don't, you can use an image editing program to scale up the image to 4× the size (twice in every direction). This will make the file much bigger and not increase quality at all — but should fool the artificial restrictions.