Problems sending and receiving messages iPhone

So I recently upgraded to an Iphone 4 about a month and a half ago. Everything was going great until about 2 days ago when the person I talked to most stopped receiving my text messages. In turn I also stopped receiving hers, however it was like a hit and miss kind of thing. For a little bit of time messages would go through perfectly fine, then suddenly it would just stop and no message would get through to either of us. Im with Bell and the person Im texting is with Rogers.
Im wondering if anyone else has experienced this type of problem? And if so how did you resolve it?
Mac