Do you know if I can use the verification call in cases where I know only the mobile number of the signatory?

The verify call checks if a signatory is or isn't registered for the electronic signature. If you choose the pseudonym, you only need the mobile number and country of the signatory. More information: https://documents.swisscom.com/product/filestore/lib/5f4322cf-3530-4d6a-b26f-b8f685f8d069/verifyid4signing-en.pdf