Answering them will cost if you are roaming, but allowing them to go to voicemail will not cost - the call is treated as a US call since it is answered by the system, not by the phone.

Some phones and service providers enable number blocking including all unknown number blocking (for example, Samsungs on T-Mobile). Blocking all unknown numbers while roaming abroad is one way to avoid excessive roaming charges, if the phone supports this.