A fugitive warrant transfer handled by the US Marshals means your boyfriend is likely in federal custody, which makes the Bureau of Prisons inmate locator your best starting point. Search for him at bop.gov/inmateloc using his name and date of birth. If he has been processed into federal custody anywhere in the system, that database will show his current location.
Two weeks of silence after a single call is consistent with what happens during an interstate transfer handled by the Marshals. The process involves moving through one or more holding facilities before reaching the destination in Pennsylvania, and communication during transit is extremely limited. Once he arrives at the receiving facility and completes intake processing, phone access typically restored within a few days to a couple of weeks.
If he does not appear in the BOP locator, the next step is checking the Pennsylvania state inmate search through the Pennsylvania Department of Corrections at cor.pa.gov. A fugitive warrant from Pennsylvania could mean he is being held in a state facility rather than a federal one, depending on the nature of the original charges.
If neither search produces results, call the US Marshals Service district office in Pennsylvania and ask about the status of the transfer. They can confirm which facility he was delivered to, which gives you a direct number to call for a status update.
The silence is almost certainly logistical rather than anything more serious. The system moves people quietly, and communication catches up once the destination facility processes them fully.