Well, phones mostly use Mini-SD cards for storage, so you're probably not going to get any more storage than you already get with your memory cards - they usually don't have card readers built in either, so you're still going to be hooking up to your laptop to transfer files to it.
You'd need to look at some form of portable image storage device rather than a phone. Something that has a hard drive inside it as well as appropriate memory card readers (some have an LCD for reviewing/deleting images, and some support various RAW file formats).
Had you considered just buying more memory cards? Probably your cheapest option.