This is a very good question and one I am pleased to see you are asking before embarking on your wireless design. Many times we see cellular products designed without consideration here, only to then fail approval.
Do you plan to have a single device/design for both European and US market? The US is typically harder to pass because you need both PTCRB and then the selected network approval. However, AT&T recently reduced their requirements with products having a ground plane less than 107mm I believe.
Its worth mentioning just in case. US carriers no longer approve 3G cellular based products. It has to be 4G/LTE. This is not the case for Europe.
Network approvals and designing a cellular product to pass is a challenge, especially when products are getting smaller and smaller. Its an important subject that should be researched before embarking on a design. Failing network approval is both costly to re-test, as well as the possibility that you may have to redesign your product.
Select your cellular wireless module carefully. Many vendors do have approval on their modules, but this does not automatically mean you do not need to approve your device because of this.