I have a UK number from Zadarma, I have connected it to VAPI (added the SIP Trunk Credentials and imported the phone number). When I tested it calling a Greek number from this phone it worked as expected. (example call ID: bc24e9bd-6423-4e6b-89a0-c68d5c5f7734) When I made a call to UK numbers it always ends receiving this automated message from user: "These calls are only possible within the country." (example call ID: bf70d462-c62c-4e0f-9021-594cbc98c163) How can I fix this?

This is most likely a carrier-level routing restriction, and looking at the logs further confirms this suspicion since the call successfully connected with the number for a few seconds. I suggest you reach out to Zadarma support and ask if UK Domestic calls need a separate routing configuration, if your account is configured for UK geographic/mobile termination, and also ask for additional logs from their side to see what happened during the last call id you provided. Have them analyze the PCAP file from your last call.