Salesforce integration - Set Phone verification automatically

Hello together, we want to integrate adobe sign into Salesforce. It is already set up and now we want to automate stuff. For example it is important to use phone as verification method.

When we create the agreement via flow/apex, we can already set a recipient by contact/user id, but verification method is email. We want to change it automatically to phone. But how? Inserting recipient records does not work (they will be created once we send out the agreement). 

And doing it manually is way to error prone. Adobe is not able to recognize phone numbers correctly. 
Example:  '+491612345678' is split to country  '+491' and number  '612345678' (should be '+49' and '1612345678')

or '+49(0)1612345678' is split to country '+(0)1612345678' and number '49'. so totally useless

My goal: In apex I create the agreement, assign the recipients (contact or user), set verification method to 'Phone' and fill country code and number automatically (phone number format is easy to split)