>But the only necessity for a fax to work is the telephone number. A web service or correctly interpreted XML has to have not only the 'address' but also the content (in term of 'field' names) matching.

It helps, but it is not required. There are tools such as the XSLT (or using the BizTalk Mapper that does this visually and generates XSLT) to map one XML document schema to another matching elements and attributes, if needed.