re: What to do after initial phone interview?

There are a number of things likely happening right now --

1). Just because it's past January 1st doesn't mean "the new year has come and gone". Never underestimate how long the HR process takes.

2). The agency/company found another candidate that they like and are still interviewing them. That candidate probably was on vacation so the agency/company had/has to wait on the candidate's availability.

3). The company and agency are negotiating fees related your recruitment. Often times the fee is negotiated up front, but when the agency goes to close and presents them with a $20,000* PO, companies start to re-evaluate and exhaust all options.

*That's a random number. The industry standard is 20% of your first year salary.

I was in recruitment for a long time and that's the list of things that came to mind when you mentioned your situation, in order of what I believe is happening.