Good afternoon, Inna.
The "Take a number from an additional field" setting works a little differently.
The system has the ability to create additional fields (system settings - other - and there it is possible to create additional fields for each entity). You use the standard "Call" field from the contact card, and it cannot be selected in this setting.
There are several solutions:
1. You create an additional field and indicate the phone number in it, in the action you also select the created additional field.
2. In the process, display the field
http://prntscr.com/ute8abIn the action, you indicate that the SMS will be sent to the client, then the phone number that is indicated in the field that I indicated above will be taken.