We are also looking to take over the builders mortgage with Banco Popular, and are experiencing the same issue with contacting them. We have even asked our solicitor to help.
We have now progressed to sending all the required details by e-mail and are now awaitng a yes or no or require more information.
The branch we a dealing with is and the contact Margarita
Avenida Fontes 10
30700 Torre-Pacheco, Spain
+34 968 578 080
Tom and Jane