First of all, I would sit down and write the business requirements and specifications. It really doesn't matter what tech you use - proper planning will affect your project timeline more than technology choice. This is especially true for an in-house custom built app.
As far as development, I would take the requirements and lay out the backend functionality. I would actually implement the backend in WCF, regardless of the client technology - that way you could use best of both worlds if needed (for example for phone integration you could write a stand-alone WPF app). ASP.NET with jQuery can easily use WCF services (JSON or XML version) together with desktop client.
As far as development of the client forms, this highly depends on developers experience and your future plans. I am not going to go into advantages/disadvantages of developing web software here - there are a ton of articles in the last 10 years about cloud/web based software (for example salesforce). I would rather concentrate on deliverables - what is your team most comfortable with today and in the future. There's a huge difference between WPF and web development, from development standpoint, and it requires completely different experience.
