ITFS numbers have some limitations, especially since each country has its own version of this type of phone number. One of those limitations is the type of phone that can dial your number. Most international toll free numbers allow calls from mobile, landline, and VoIP but others may only allow incoming calls from one or some of these options.

Solution #2: 

Various factors define the accessibility of ITFS numbers through mobile networks and VoIP.  Country regulations, carrier mandates, and telecommunications providers determine whether callers are able to access your toll free number using a mobile phone, or through an Internet Protocol (IP) address. For example, Denmark toll free phone numbers are accessible from mobile networks at premium rate while the United Kingdom toll free numbers have mobile access with no additional charge, but they are not accessible from payphones.